Improving the development of early bovine somatic-cell nuclear transfer embryos by treating adult donor cells with vitamin C

Early Cell Specification in Mammalian Fertilized and Somatic Cell Nuclear Transfer Embryos

Early cell specification in mammalian preimplantation embryos is an intricate cellular process that leads to coordinated spatial and temporal expression of specific genes. Proper segregation into the first two cell lineages, the inner cell mass (ICM) and the trophectoderm (TE), is imperative for developing the embryo proper and the placenta, respectively. Somatic cell nuclear transfer (SCNT) allows the formation of a blastocyst containing both ICM and TE from a differentiated cell nucleus, which means that this differentiated genome must be reprogrammed to a totipotent state. Although blastocysts can be generated efficiently through SCNT, the full-term development of SCNT embryos is impaired mostly due to placental defects. In this review, we examine the early cell fate decisions in fertilized embryos and compare them to observations in SCNT-derived embryos, in order to understand if these processes are affected by SCNT and could be responsible for the low success of reproductive cloning.

Ascorbic acid protects the toxic effects of aflatoxin B1 on yak oocyte maturation

High-quality oocytes are a prerequisite for successful fertilization. Mammals feeding on aflatoxin-contaminated feed can cause reproductive toxicity, including follicular atresia, poor oocyte development and maturation, and aberrant epigenetic modifications of oocytes. In addition, the important role of ascorbic acid (AA) in reproductive biology has been confirmed, and AA is widely used as an antioxidant in cell culture. However, the toxic effects of aflatoxin B₁ (AFB₁ ) on yak oocytes and whether AA has protective effects remain unknown. In this study, we found that exposure to AFB₁ impedes meiotic maturation of oocytes, promotes apoptosis by triggering high levels of reactive oxygen species (ROS), and disrupts mitochondrial distribution and actin integrity, resulting in a decrease in the fertilization ability and parthenogenetic development ability of oocytes. In addition, these injuries changed the DNA methylation transferase transcription level of mature oocytes. After adding 50 μg/ml AA, the indices recovered to levels close to those of the control group. The results showed that AA could protect yak oocytes from the toxic effects of AFB₁ and improve the quality of oocytes.

Strategies to Improve the Efficiency of Somatic Cell Nuclear Transfer

Mammalian oocytes can reprogram differentiated somatic cells into a totipotent state through somatic cell nuclear transfer (SCNT), which is known as cloning. Although many mammalian species have been successfully cloned, the majority of cloned embryos failed to develop to term, resulting in the overall cloning efficiency being still low. There are many factors contributing to the cloning success. Aberrant epigenetic reprogramming is a major cause for the developmental failure of cloned embryos and abnormalities in the cloned offspring. Numerous research groups attempted multiple strategies to technically improve each step of the SCNT procedure and rescue abnormal epigenetic reprogramming by modulating DNA methylation and histone modifications, overexpression or repression of embryonic-related genes, etc. Here, we review the recent approaches for technical SCNT improvement and ameliorating epigenetic modifications in donor cells, oocytes, and cloned embryos in order to enhance cloning efficiency.

Interleukin 17D Enhances the Developmental Competence of Cloned Pig Embryos by Inhibiting Apoptosis and Promoting Embryonic Genome Activation

Simple Summary

The cloning technique is important for animal husbandry and biomedicine because it can be used to clone superior breeding livestock and produce multipurpose genetically modified animals. However, the success rate of cloning currently is very low due to the low developmental efficiency of cloned embryos, which limits the application of cloning. The low developmental competence is related to the excessive cell death in cloned embryos. Interleukin 17D (IL17D) is required for the normal development of mouse embryos by inhibiting cell death. This study aimed to investigate whether IL17D can improve cloned pig embryo development by inhibiting cell death. Addition of IL17D protein to culture medium decreased the cell death level and improved the developmental ability of cloned pig embryos. IL17D treatment enhanced cloned pig embryo development by regulating cell death-associated gene pathways and promoting genome-wide gene expression, which is probably via up-regulating the expression of a gene called GADD45B. This study provided a new approach to improve the pig cloning efficiency by adding IL17D protein to the culture medium of cloned pig embryos.

Abstract

Cloned animals generated by the somatic cell nuclear transfer (SCNT) approach are valuable for the farm animal industry and biomedical science. Nevertheless, the extremely low developmental efficiency of cloned embryos hinders the application of SCNT. Low developmental competence is related to the higher apoptosis level in cloned embryos than in fertilization-derived counterparts. Interleukin 17D (IL17D) expression is up-regulated during early mouse embryo development and is required for normal development of mouse embryos by inhibiting apoptosis. This study aimed to investigate whether IL17D plays roles in regulating pig SCNT embryo development. Supplementation of IL17D to culture medium improved the developmental competence and decreased the cell apoptosis level in cloned porcine embryos. The transcriptome data indicated that IL17D activated apoptosis-associated pathways and promoted global gene expression at embryonic genome activation (EGA) stage in treated pig SCNT embryos. Treating pig SCNT embryos with IL17D up-regulated expression of GADD45B, which is functional in inhibiting apoptosis and promoting EGA. Overexpression of GADD45B enhanced the developmental efficiency of cloned pig embryos. These results suggested that IL17D treatment enhanced the developmental ability of cloned pig embryos by suppressing apoptosis and promoting EGA, which was related to the up-regulation of GADD45B expression. This study demonstrated the roles of IL17D in early development of porcine SCNT embryos and provided a new approach to improve the developmental efficiency of cloned porcine embryos.

SVCT2 Overexpression and Ascorbic Acid Uptake Increase Cortical Neuron Differentiation, Which Is Dependent on Vitamin C Recycling between Neurons and Astrocytes

During brain development, sodium–vitamin C transporter (SVCT2) has been detected primarily in radial glial cells in situ, with low-to-absent expression in cerebral cortex neuroblasts. However, strong SVCT2 expression is observed during the first postnatal days, resulting in increased intracellular concentration of vitamin C. Hippocampal neurons isolated from SVCT2 knockout mice showed shorter neurites and low clustering of glutamate receptors. Other studies have shown that vitamin C-deprived guinea pigs have reduced spatial memory, suggesting that ascorbic acid (AA) and SVCT2 have important roles in postnatal neuronal differentiation and neurite formation. In this study, SVCT2 lentiviral overexpression induced branching and increased synaptic proteins expression in primary cultures of cortical neurons. Analysis in neuroblastoma 2a (Neuro2a) and human subventricular tumor C3 (HSVT-C3) cells showed similar branching results. SVCT2 was mainly observed in the cell membrane and endoplasmic reticulum; however, it was not detected in the mitochondria. Cellular branching in neuronal cells and in a previously standardized neurosphere assay is dependent on the recycling of vitamin C or reduction in dehydroascorbic acid (DHA, produced by neurons) by glial cells. The effect of WZB117, a selective glucose/DHA transporter 1 (GLUT1) inhibitor expressed in glial cells, was also studied. By inhibiting GLUT1 glial cells, a loss of branching is observed in vitro, which is reproduced in the cerebral cortex in situ. We concluded that vitamin C recycling between neurons and astrocyte-like cells is fundamental to maintain neuronal differentiation in vitro and in vivo. The recycling activity begins at the cerebral postnatal cortex when neurons increase SVCT2 expression and concomitantly, GLUT1 is expressed in glial cells.

Procyanidin B1 promotes in vitro maturation of pig oocytes by reducing oxidative stress

Oxidative stress negatively affects the in vitro maturation (IVM) of oocytes. Procyanidin B1 (PB1) is a natural polyphenolic compound that has antioxidant properties. In this study, we investigated the effect of PB1 supplementation during IVM of porcine oocytes. Treatment with 100 μM PB1 significantly increased the MII oocytes rate (p <0.05), the parthenogenetic (PA) blastocyst rate (p <0.01) and the total cell number in the PA blastocyst (p < 0.01) which were cultured in regular in vitro culture (IVC) medium. The PA blastocyst rate of regular MII oocytes activated and cultured in IVC medium supplemented with 100 and 150 μM PB1 significantly increased compared with control (p < 0.01 and p < 0.05). We also evaluated the reactive oxygen species (ROS) levels, mitochondrial membrane potential (Δψm) levels, glutathione (GSH) levels, and apoptotic levels in MII oocytes and cumulus cells following 100 μM PB1 treatment. The results showed that the PB1 supplementation decreased ROS production and apoptotic levels. In addition, PB1 was found to increase Δψm levels and GSH levels. In conclusion, PB1 inhibited apoptosis of oocytes and cumulus cells by reducing oxidative stress. Moreover, PB1 improved the quality of oocytes and promoted PA embryo development. Taken together, our results suggest that PB1 is a promising antioxidant additive for IVM of oocytes.

Vitamin C protects early mouse embryos against juglone toxicity

Juglone, a naphthoquinone isolated from many species of the Juglandaceae (walnut) family, has been used in traditional Chinese medicine for centuries for its various pharmacological effects. Our previous research found its toxic effects on oocytes maturation. But we still know a little about its toxic effects on embryo development. Here, we used mouse embryo as a model to explore the effects of juglone on early mammalian embryo development. Exposure to juglone significantly decreased the development rate in early mouse embryos in vitro. Moreover, juglone exposure led to developmental arrest by disturbing mitochondrial function, producing abnormal epigenetic modifications, inducing high levels of oxidative stress and DNA damage, and increasing the rate of embryonic cell apoptosis. However, vitamin C (VC) ameliorated the toxic effects of juglone to a certain extent. Overall, juglone has a toxic effect on early embryo development through the generation of ROS and apoptosis. But VC was able to protect against these juglone-induced defects. These results not only give a new perspective on juglone's pharmacological effects on early mammalian embryo development, but also provide ideas for the better application of this agent in traditional Chinese medicine.

RNA sequencing revealed the abnormal transcriptional profile in cloned bovine embryos

Somatic cell nuclear transfer (SCNT) has potential applications in agriculture and biomedicine, but the efficiency of cloning is still low. In this study, the transcriptional profiles in cloned and fertilized embryos were measured and compared by RNA sequencing. The 2-cell embryos were detected to identify the earliest transcriptional differences between embryos derived through IVF and SCNT. As a result, 364 genes showed decreased expression in cloned 2-cell embryos and were enriched in "intracellular protein transport" and "ubiquitin mediated proteolysis". In blastocysts, 593 genes showed decreased expression in cloned blastocysts and were enriched in "RNA binding", "nucleotide binding", "embryo development", and "adherens junction". We identified 14 development related genes that were not activated in the cloned embryos. Then, 68 and 245 long non-coding RNAs were recognized abnormally expressed in cloned 2-cell embryos and cloned blastocysts, respectively. Furthermore, we found that incomplete RNA-editing occurred in cloned embryos and might be caused by decreased ADAR expression. In conclusion, our study revealed the abnormal transcripts and deficient RNA-editing sites in cloned embryos and provided new data for further mechanistic studies of somatic nuclear reprogramming.

Vitamin C treatment of embryos, but not donor cells, improves the cloned embryonic development in sheep

Vitamin C is not only an antioxidant but also a regulator of epigenetic modifications that can enhance the activity of the ten-eleven translocation (TET) family dioxygenases and promote the oxidation of 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC). Here, we investigated the effects of vitamin C in regulating DNA methylation in sheep somatic cells or embryos in an effort to improve the cloned embryo development. Vitamin C treatment of sheep foetal fibroblast cells significantly increased the 5hmC levels but did not affect the 5mC levels in cells. After nuclear transfer, vitamin C-treated donor cells could not support a higher blastocyst development rate than non-treated cells. Although combination of serum starvation and vitamin C treatment could induce significant 5mC decrease in donor cells, it failed to promote the development of resultant cloned embryos. When cloned embryos were directly treated with vitamin C, the pre-implantation development of embryos and the 5hmC levels in blastocysts were significantly improved. This beneficial role of vitamin C on embryo development was also observed in fertilized embryos. Our results suggest that vitamin C treatment of the embryos, but not the donor cells, can improve the development of cloned sheep embryos.

DNA methylation regulated by ascorbic acids in yak preimplantation embryo helps to improve blastocyst quality

DNA methylation as an important, essential epigenetic modification is critical for the successful development of mammalian embryos. In recent years, the important role of ascorbic acid (AA) as an irreplaceable cofactor for epigenetic regulation has been confirmed. However, the effect of AA on DNA methylation in preimplantation embryo development of plateau yak remains unknown. In this study, we explored whether AA can help regulates DNA methylation in yak preimplantation embryos to improve the blastocyst quality. First, our results indicate that the preimplantation of the yak still follows the classical pattern of DNA demethylation and remethylation, however, remethylation occurs in the blastocyst stage. Second, the unique expression pattern of the ten-eleven translocation enzyme (TET3) in the cytoplasm plays a key role in the demethylation mechanism. Third, in the blastocyst stage, the pluripotency gene CDX2 promoter region was in a hypomethylated state, and the POU5F1, SOX2, and NANOG promoter regions were in moderate methylation states. In addition, treatment with 50 μg/ml AA mainly improved the expression levels of DNMT1, DNMT3a, and TET3, ensured the establishment, maintenance and transition of 5-methylcytosine. After AA treatment, the methylation level of the pluripotency genes NANOG promoter regions was significantly reduced, and the mRNA transcript abundance of the pluripotency genes NANOG, POU5F1, and CDX2 was upregulated. In conclusion, our findings suggest that AA could increase blastocyst cell numbers by regulating DNA methylation of yak preimplantation embryos .

Porcine Fetal-Derived Fibroblasts Alter Gene Expression and Mitochondria to Compensate for Hypoxic Stress During Culture

The Warburg effect is characterized by decreased mitochondrial oxidative phosphorylation and increased glycolytic flux in adequate oxygen. The preimplantation embryo has been described to have characteristics of the Warburg effect, including similar changes in gene expression and mitochondria, which are more rudimentary in appearance. We hypothesized hypoxia would facilitate anaerobic glycolysis in fibroblasts thereby promoting gene expression and media metabolite production reflecting the Warburg effect hallmarks in early embryos. Additionally, we speculated that hypoxia would induce a rudimentary small mitochondrial phenotype observed in several cell types evidenced to demonstrate the Warburg effect. While many have examined the role hypoxia plays in pathological conditions, few studies have investigated changes in primary cells which could be used in somatic cell nuclear transfer. We found that cells grown in 1.25% O₂ had normal cell viability and more, but smaller mitochondria. Several hypoxia-inducible genes were identified, including seven genes for glycolytic enzymes. In conditioned media from hypoxic cells, the quantities of gluconolactone, cytosine, and uric acid were decreased indicating higher consumption than control cells. These results indicate that fibroblasts alter gene expression and mitochondria to compensate for hypoxic stress and maintain viability. Furthermore, the metabolic changes observed, making them more similar to preimplantation embryos, could be facilitating nuclear reprogramming making these cells more amendable to future use in somatic cell nuclear transfer.

Transcriptome Analyses Reveal Effects of Vitamin C-Treated Donor Cells on Cloned Bovine Embryo Development

Somatic cell nuclear transfer (SCNT) is a very powerful technique used to produce genetically identical or modified animals. However, the cloning efficiency in mammals remains low. In this study, we aimed to explore the effects of vitamin C (Vc)-treated donor cells on cloned embryos. As a result, Vc treatment relaxed the chromatin of donor cells and improved cloned embryo development. RNA sequencing was adopted to investigate the changes in the transcriptional profiles in early embryos. We found that Vc treatment increased the expression of genes involved in the cell–substrate adherens junction. Gene ontology (GO) analysis revealed that Vc treatment facilitated the activation of autophagy, which was deficient in cloned two-cell embryos. Rapamycin, an effective autophagy activator, increased the formation of cloned blastocysts (36.0% vs. 25.6%, p < 0.05). Abnormal expression of some coding genes and long non-coding RNAs in cloned embryos was restored by Vc treatment, including the zinc-finger protein 641 (ZNF641). ZNF641 compensation by means of mRNA microinjection improved the developmental potential of cloned embryos. Moreover, Vc treatment rescued some deficient RNA-editing sites in cloned two-cell embryos. Collectively, Vc-treated donor cells improved the development of the cloned embryo by affecting embryonic transcription. This study provided useful resources for future work to promote the reprogramming process in SCNT embryos.

Resveratrol improved the developmental potential of oocytes after vitrification by modifying the epigenetics

Resveratrol (Res) has been reported to be able to improve oocyte vitrification because of its antioxidative properties. The objective of this study was to further assess the positive effect of Res addition on the developmental potential of vitrified mouse oocytes from the perspective of epigenetic alterations. First, 2 μM Res was chosen as the optimal concentration on the basis of its effects on survival and its antioxidative properties. We found that Res addition significantly promoted fertilization (63.8% vs. 42.9%) and blastocyst formation (68.3% vs. 50.2%) after oocyte vitrification. The quality of the derived blastocysts was also higher after Res treatment. Regarding epigenetic aspects, the expression of the important deacetylase SIRT1 was found to decrease significantly upon vitrification, but it was rescued by Res. The abnormal levels of H3K9 acetylation and DNA methylation in vitrified oocytes were restored by Res addition. Moreover, the expression of several imprinted genes was affected by oocyte vitrification. Among them, abnormal Gtl2 and Peg3 expression levels were restored by Res addition. Therefore, the methylation of their imprinted control regions (ICRs) was examined. Surprisingly, the abnormal patterns of Gtl2 and Peg3 methylation in blastocysts developed from vitrified oocytes were both restored by Res addition. Finally, the full-term embryonic development showed that the birth rate was improved significantly by Res addition (56.2% vs. 38.1%). Collectively, Res was beneficial for the pre- and postimplantation embryonic development. Except for the antioxidative activity, Res also played a role in the correction of some abnormal epigenetic modifications caused by oocyte vitrification.

Overexpression of miR-101-2 in donor cells improves the early development of Holstein cow somatic cell nuclear transfer embryos

Accumulating studies have suggested that microRNA play a part in regulating multiple cellular processes, such as cell proliferation, apoptosis, the cell cycle, and embryo development. This study explored the effects of miR-101-2 on donor cell physiological status and the development of Holstein cow somatic cell nuclear transfer (SCNT) embryos in vitro. Holstein cow bovine fetal fibroblasts (BFF) overexpressing miR-101-2 were used as donor cells to perform SCNT; then, cleavage rate, blastocyst rate, inner cell mass-to-trophectoderm ratio, and the expression of some development- and apoptosis-related genes in different groups were analyzed. The miR-101-2 suppressed the expression of inhibitor of growth protein 3 (ING3) at mRNA and protein levels, expedited cell proliferation, and decreased apoptosis in BFF, suggesting that ING3, a target gene of miR-101-2, is a potential player in this process. Moreover, by utilizing donor cells overexpressing miR-101-2, the development of bovine SCNT embryos in vitro was significantly enhanced; the apoptotic rate in SCNT blastocysts was reduced, and the inner cell mass-to-trophectoderm ratio and SOX2, POU5F1, and BCL2L1 expression significantly increased, whereas BAX and ING3 expression decreased. Collectively, these findings suggest that miR-101-2 promotes BFF proliferation and vitality, reduces their apoptosis, and improves the early development of SCNT embryos.

Improvement of in vitro and early in utero porcine clone development after somatic donor cells are cultured under hypoxia

Genetically engineered pigs serve as excellent biomedical and agricultural models. To date, the most reliable way to generate genetically engineered pigs is via somatic cell nuclear transfer (SCNT), however, the efficiency of cloning in pigs is low (1-3%). Somatic cells such as fibroblasts frequently used in nuclear transfer utilize the tricarboxylic acid cycle and mitochondrial oxidative phosphorylation for efficient energy production. The metabolism of somatic cells contrasts with cells within the early embryo, which predominately use glycolysis. We hypothesized that fibroblast cells could become blastomere-like if mitochondrial oxidative phosphorylation was inhibited by hypoxia and that this would result in improved in vitro embryonic development after SCNT. In a previous study, we demonstrated that fibroblasts cultured under hypoxic conditions had changes in gene expression consistent with increased glycolytic/gluconeogenic metabolism. The goal of this pilot study was to determine if subsequent in vitro embryo development is impacted by cloning porcine embryonic fibroblasts cultured in hypoxia. Here we demonstrate that in vitro measures such as early cleavage, blastocyst development, and blastocyst cell number are improved (4.4%, 5.5%, and 17.6 cells, respectively) when donor cells are cultured in hypoxia before nuclear transfer. Survival probability was increased in clones from hypoxic cultured donors compared to controls (8.5 vs. 4.0 ± 0.2). These results suggest that the clones from donor cells cultured in hypoxia are more developmentally competent and this may be due to improved nuclear reprogramming during somatic cell nuclear transfer.

Pharmacologic treatment of donor cells induced to have a Warburg effect-like metabolism does not alter embryonic development in vitro or survival during early gestation when used in somatic cell nuclear transfer in pigs

Somatic cell nuclear transfer is a valuable technique for the generation of genetically engineered animals, however, the efficiency of cloning in mammalian species is low (1-3%). Differentiated somatic cells commonly used in nuclear transfer utilize the tricarboxylic acid cycle and cellular respiration for energy production. Comparatively the metabolism of somatic cells contrasts that of the cells within the early embryos which predominately use glycolysis. Early embryos (prior to implantation) are evidenced to exhibit characteristics of a Warburg Effect (WE)-like metabolism. We hypothesized that pharmacologically driven fibroblast cells can become more blastomere-like and result in improved in vitro embryonic development after SCNT. The goals were to determine if subsequent in vitro embryo development is impacted by (1) cloning pharmacologically treated donor cells pushed to have a WE-like metabolism or (2) culturing non-treated donor clones with pharmaceuticals used to push a WE-like metabolism. Additionally, we investigated early gestational survival of the donor-treated clone embryos. Here we demonstrate that in vitro development of clones is not hindered by pharmacologically treating either the donor cells or the embryos themselves with CPI, PS48, or the combination of these drugs. Furthermore, these experiments demonstrate that early embryos (or at least in vitro produced embryos) have a low proportion of mitochondria which have high membrane potential and treatment with these pharmaceuticals does not further alter the mitochondrial function in early embryos. Lastly, we show that survival in early gestation was not different between clones from pharmacologically induced WE-like donor cells and controls.

Supplement of autologous ooplasm into porcine somatic cell nuclear transfer embryos does not alter embryo development

Somatic cell nuclear transfer (SCNT) is considered as the technique in which a somatic cell is introduced into an enucleated oocyte to make a cloned animal. However, it is unavoidable to lose a small amount of the ooplasm during enucleation step during SCNT procedure. The present study was aimed to uncover whether the supplement of autologous ooplasm could ameliorate the oocyte competence so as to improve low efficiency of embryo development in porcine SCNT. Autologous ooplasm-transferred (AOT) embryos were generated by the supplementation with autologous ooplasm into SCNT embryos. They were comparatively evaluated with respect to embryo developmental potential, the number of apoptotic body formation and gene expression including embryonic lineage differentiation, apoptosis, epigenetics and mitochondrial activity in comparison with parthenogenetic, in vitro-fertilized (IVF) and SCNT embryos. Although AOT embryos showed perfect fusion of autologous donor ooplasm with recipient SCNT embryos, the supplement of autologous ooplasm could not ameliorate embryo developmental potential in regard to the rate of blastocyst formation, total cell number and the number of apoptotic body. Furthermore, overall gene expression of AOT embryos was presented with no significant alterations in comparison with that of SCNT embryos. Taken together, the results of AOT demonstrated inability to make relevant values improved from the level of SCNT embryos to their IVF counterparts.