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Artificial oocyte activation is an essential step in somatic cell nuclear transfer (SCNT) and can enhance viability of embryos as a form of assisted reproductive technology (ART) in clinics. Most artificial activation methods have been developed to increase cytosolic calcium (Ca2+) level in oocytes. Interestingly, recent studies have demonstrated that mammalian oocytes can be activated using N,N,N′,N′-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), a Zn2+ chelator. Although effective, TPEN is also known to induce apoptosis and shows poor selectivity between free Zn2+ and protein-bound Zn2+. The aim of this study was to identify different Zn2+ chelators that can activate pig oocytes. Among five Zn2+ chelators examined, 1,10-phenanthroline (Phen), and tris(2-pyridylmethyl)amine (TPA) successfully activated pig oocytes. The level of available Zn2+ was reduced without any increase in Ca2+ in oocytes incubated with Phen or TPA, indicating that the oocyte activation occurred independently of Ca2+ signal. When various concentrations (100–500 μM) and incubation durations (10–120 min) of Phen and TPA were used to activate pig oocytes, 500 μM for 60 min and 100 μM for 60 min of Phen and TPA treatments, respectively, were found to be most effective in supporting embryo development. The frequency of blastocyst formation after the treatments was higher than 40% at day 7. When oocytes were incubated with TPEN, Phen, or TPA under their optimal treatment conditions, there was no significant difference in the frequencies of day 7 blastocyst formation among the three treatments. However, day 5 blastocyst formation was observed from the Phen- and TPA-treated oocytes, whereas no blastocyst was formed at day 5 in the TPEN-treated oocytes. The average total cell number in day 7 blastocysts was higher in the Phen treatment group than in the TPEN treatment (P

Historically, genetic engineering in livestock proved to be challenging. Without stable embryonic stem cell lines to utilize, somatic cell nuclear transfer (SCNT) had to be employed to produce many of the genetically engineered (GE) livestock models. Through the genetic engineering of somatic cells followed by SCNT, GE livestock models could be generated carrying site-specific modifications. Although successful, only a few GE livestock models were generated because of low efficiency and associated birth defects. Recently, there have been major strides in the development of genome editing tools: Zinc-Finger Nucleases (ZFNs), Transcription activator-like effector nucleases (TALENS), and Clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated 9 (Cas9) system. These tools rely on the generation of a double strand DNA break, followed by one of two repair pathways: non-homologous end joining (NHEJ) or homology directed repair (HDR). Compared to the traditional approaches, these tools dramatically reduce time and effort needed to establish a GE animal. Another benefit of utilizing genome editing tools is the application of direct injection into developing embryos to induce targeted mutations, therefore, eliminating side effects associated with SCNT. Emerging technological advancements of genome editing systems have dramatically improved efficiency to generate GE livestock models for both biomedical and agricultural purposes. Although the efficiency of genome editing tools has revolutionized GE livestock production, improvements for safe and consistent application are desired. This review will provide an overview of genome editing techniques, as well as examples of GE livestock models for agricultural and biomedical purposes.

Traditionally, genetic engineering in the pig was a challenging task. Genetic engineering of somatic cells followed by somatic cell nuclear transfer (SCNT) could produce genetically engineered (GE) pigs carrying site-specific modifications. However, due to difficulties in engineering the genome of somatic cells and developmental defects associated with SCNT, a limited number of GE pig models were reported. Recent developments in genome-editing tools, such as zinc finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs) and the clustered regularly interspaced short palindromic repeat (CRISPR)/CRISPR-associated (Cas) 9 system, have markedly changed the effort and time required to produce GE pig models. The frequency of genetic engineering in somatic cells is now practical. In addition, SCNT is no longer essential in producing GE pigs carrying site-specific modifications, because direct injection of genome-editing systems into developing embryos introduces targeted modifications. To date, the CRISPR/Cas9 system is the most convenient, cost-effective, timely and commonly used genome-editing technology. Several applicable biomedical and agricultural pig models have been generated using the CRISPR/Cas9 system. Although the efficiency of genetic engineering has been markedly enhanced with the use of genome-editing systems, improvements are still needed to optimally use the emerging technology. Current and future advances in genome-editing strategies will have a monumental effect on pig models used in agriculture and biomedicine.

Fertilized oocytes undergo genome-wide DNA demethylation with the exception of imprinted genes and certain repetitive elements. Ten-eleven translocation 3 (TET3) protein has been known to be responsible for the DNA demethylation process by catalysing oxidation of 5-methylcytosine. Recent studies in the mouse indicated that multiple Tet3 isoforms exist in oocytes, implying differential actions of the isoforms. Previously, we reported the sequence of TET3 in porcine oocytes (GenBank: KC137685). Here, we investigated the presence of TET3 isoforms in porcine oocytes and cumulus cells, and followed changes in the TET3 expression during oocyte maturation to further understand the mechanism of TET3 action in the DNA demethylation process. To identify porcine TET3 isoforms, 5′ RACE (rapid amplification of cDNA ends) was conducted using mRNAs isolated from oocytes and cumulus cells at both germinal vesicle (GV) and MII stages. Gene-specific primers for 5′RACE were designed to recognise conserved regions of TET3 that are present in all isoforms, based on EST databases and the cloned sequence in our previous study. The PCR-amplified 5′RACE products were cloned into a vector and subsequently sequenced. The 5′RACE revealed 3 different TET3 isoforms from GV and MII cumulus cells; no amplification was detected using oocytes, potentially due to a low amount of mRNA. Among the isoforms, the longest variant (TET3L) contained sequences for the CXXC domain, known to carry DNA binding properties. Then, RT-PCR was used to detect the presence of the isoforms in porcine oocytes. Interestingly, the expression of TET3 isoform containing the CXXC domain (TET3L) was only verified from the RT-PCR, suggesting that the isoform may be the predominant isoform in porcine oocytes. To characterise transcript abundance of porcine TET3L, RNAs were isolated from different cells and/or tissues including cumulus cells, oocytes, brain, spleen, and lung. The RT-qPCR was performed using the RNAs and ΔΔCT method was used to analyse the data; GAPDH was used as an internal control. Three biological replicates were used for analysis of RT-qPCR data and P-values of less than 0.05 from one-way analysis of variance were considered significant. The expression level of TET3L was much higher in MII oocytes compared with that in somatic tissues; MII oocytes expressed the TET3 isoform over 350-fold higher than MII cumulus cells and 18-fold higher than lung cells. Interestingly, the expression level of TET3L increased over 3-fold during oocyte maturation (i.e. from GV to MII stage oocytes), indicating that TET3L may have a significant role in DNA demethylation after fertilisation. In conclusion, the TET3 isoform containing CXXC domain (TET3L) is predominantly expressed in matured porcine oocytes, suggesting an important role of the TET3 CXXC domain in DNA demethylation in zygotes.

The objective of this study was to determine if physical parameters of bovine zygotes are correlated with invitro development success. We examined the relationship between a zygote's outer diameter (OD), cell area, and zona pellucida (ZP) thickness on blastocyst development and blastomere number. Bovine cumulus-oocyte complexes, from abattoir-derived ovaries, were matured in tissue culture medium 199-based maturation medium for 21-23h. Invitro fertilisation was performed with frozen thawed semen containing a pool of 4 Holstein bulls. After incubation with sperm for 16-18h, presumptive zygotes (n=875) were denuded and placed in 5µL droplets of synthetic oviductal fluid-bovine embryo 1 medium (SOF-BE1) under oil to be individually photographed using an inverted scope with a digital camera. Zygotes were then group-cultured in 50µL of synthetic oviductal fluid-BE1 droplets in a polyester micromesh to identify individuals. Development to the blastocyst stage was recorded on Day 7 and Day 8 of culture, and Day 8 embryos were fixed and stained with Hoechst 33342 (n=87). ImageJ was used to measure the OD, area, and ZP from images. Data were analysed with a logistic regression using the lme4 package in R with Day 7 or Day 8 blastocyst development as the response; OD, area, and ZP as predictors; and study replicate (n=11) and culture droplet as random effects. Residual estimates of area and ZP calculated from OD were used to account for collinearity. Average OD was 151.2µm, ranging 130.4-171.6µm. Average ZP was 11.8µm, ranging 7.2-17.5µm. Area averaged 9856.7µm2, ranging from 6421.9 to 13 814.8 µm2. There was a tendency for an effect of OD on probability of development on Day 8 (P=0.08) but not Day 7. There was an effect of ZP on the probability of development on Day 8 (P=0.04) but not Day 7. Blastocyst rates on Day 8 for zygotes with ZP 11.8µm were 21.7%. There was an interaction (P12.7µm and OD >155.8µm averaged 43.3%. The observed blastocyst rates for these two groups were 26.2% and 25%, respectively. Zygotes with OD between 146.9-155.8µm and ZP between 10.7-12.7µm had an estimated development probability of 22.2%. The obtained Day 8 blastocyst rate for this group was 22.5%. Area did not affect probability of development, but was positively correlated with total blastomere number at Day 8 (P=0.01; marginal R2=0.09). The observed interaction between ZP thickness and OD may be indicative of an optimum ooplasm area before maturation or fertilisation that may enhance development. Area assessment after fertilisation is not correlated with development success. In addition, these data in comparison with our previous data generated in parthenogenetic embryos indicate that artificial activation of oocytes is not an ideal model in place of IVF for studying development. We continue to demonstrate that physical parameters of zygotes may have potential as a noninvasive, objective selection tool of embryos.

Establishing proper levels of pluripotency is essential for normal development. The genome of gametes is remodelled upon fertilisation and pluripotency-related genes are expressed in blastocysts. Multiple pluripotency-related genes are involved in the well-orchestrated process; however, detailed mechanistic actions remain elusive. The PRDM family genes are reported to be closely related to the pluripotency. A previous report noted that PRDM14 plays an important role in the maintenance of pluripotency in human embryonic stem cells (ESCs) and potentially murine ESCs; loss of PRDM14 was found to cause abnormalities in genome-wide epigenetic status. Similarly, PRDM15 was found to be a key regulator of pluripotency in mouse ESCs. Structural similarities among the PRDM family suggest that other PRDM family genes may help to establish and maintain pluripotency in embryos. Unfortunately, little is known about the expression profile of PRDM family in porcine embryos. To expand our understanding of the role of PRDM family in porcine embryos, expression patterns of PRDM gene family were investigated using reverse transcription quantitative (RTq)-PCR. Candidate PRDM family genes were selected based on previous RNA-Seq data in porcine oocytes/embryos. To conduct this study, germinal vesicle (GV), MII, zygote, 4-cell, and blastocyst samples were collected. Complementary DNA synthesised from the samples was used for RT-qPCR to analyse the expression pattern of selected PRDM family genes: PRDM2, PRDM4, PRDM6, PRDM14, and PRDM15. The expression of target genes was normalized to the YWHAG level, an internal control. Then, GV stage was used as a control for ΔΔCT analysis. Two technical replications and three biological replications were performed. Analysis of variance was used for statistical analysis and P-values

Ten-eleven translocation (TET) methylcytosine dioxygenases are considered to play an important role in regulation of DNA methylation patterns by converting 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC). TET3 protein, a member of TET family, is enriched in mature oocytes and early stage embryos and contributes to DNA demethylation of the paternal genome in zygotes. N-terminal CXXC domain of TET3 is thought to be important in catalysing 5mC oxidation through its DNA binding potential. However, it is not clear whether specific DNA binding of CXXC domain is required for 5hmC conversion in mammalian embryos. Here, we investigated the role of TET3 CXXC domain in controlling 5hmC formation in fertilized pig embryos by injecting TET3 CXXC domain into mature pig oocytes as a dominant negative to inhibit the direct binding of TET3 to the genome through the CXXC domain. The CXXC domain of pig TET3 was identified through bioinformatics comparison of TET3 sequences among different species and cloned from mature pig oocyte-derived cDNA. To construct the green fluorescent protein (GFP)-CXXC fusion protein, CXXC sequence was subcloned into N-terminal GFP fusion vector, and then mRNA was synthesised by in vitro transcription. The GFP-CXXC mRNA (100 ng/µL) was injected into oocytes matured in vitro for 36 to 37h. Then, the oocytes were fertilized at 42h post-maturation. Water-injected oocytes served as a control. At 17h post-fertilization, zygotes were collected to analyse 5hmC level by immunocytochemistry. The level of 5hmC was analysed using ImageJ (https://imagej.nih.gov/ij/). Expression of pluripotency-related genes at Day 7 blastocysts was examined through quantitative RT-PCR; ΔΔCt method was used to analyse the quantitative RT-PCR data and Student’s t-test was used for statistical analysis. All experiments were conducted at least three times and P-values of less than 0.05 were considered significant. The GFP-CXXC was exclusively localised in pronuclei, indicating that the CXXC domain may lead to nuclear localization of TET3. The level of 5hmC in zygotes was not altered by the overexpression of GFP-CXXC. Interestingly, in 2-cell stage embryos, the 5hmC level was reduced in GFP-CXXC injected embryos compared with the control group, suggesting that CXXC domain is important for 5hmC formation post-DNA replication. There was an increase in transcript abundance of NANOG and ESRRB in blastocysts developed from GFP-CXXC injected oocytes compared with control blastocysts (P

Despite extensive efforts, cellular reprogramming in livestock species has had limited success. Induced pluripotent stem cells (iPSC) have been established; however, these cells often show incomplete reprogramming status, and constitutive expression of exogenous reprogramming factors is required due to inactivation of endogenous pluripotency-related genes. A previous study reported that overexpression of the Xenopus egg-derived WAVE1 gene assists reprogramming of murine somatic cells into the pluripotent state. The WAVE1 gene is also required for oocyte-mediated reprogramming by transcriptional activation of embryonic genes. In this study, we investigated the role of porcine WAVE1 in cellular reprogramming by inducing overexpression of WAVE1 in porcine fetal fibroblasts (PFF). Previously, we cloned the coding sequences (CDS) of porcine WAVE1 using porcine expressed sequence tags (EST) and predicted porcine WAVE1 sequences. The WAVE1 CDS, derived from porcine mature oocytes, was overexpressed in PFF by transfection using the Neon system. Then, G418-based antibiotic selection was performed to enrich cells constitutively overexpressing WAVE1. After cell culture for 4 weeks, RNA was extracted from the WAVE1 transfected and control PFF, and cDNA was synthesised from the RNA using random hexamers. The cDNAs were used for quantitative reverse transcription PCR to analyse the expression pattern of pluripotency- and reprogramming-related genes: POU5F1, NANOG, KLF2, SOX2, DPPA3, ZFP42, ESRRB, TET1, TET2, and TET3. The expression of target genes were normalized to GAPDH level and the ΔΔCt algorithm was used for analysis. Three technical replications and 4 biological replications were performed. Student’s t-test was used for the comparison and P-values

DNA methylation is one of the principal epigenetic modifications playing an essential role in regulating gene expression. The TET family (1-3) is implicated in initiating the demethylation process by converting 5-methylcytosine (5mC) to 5-hydroxymethyl cytosine (5hmC) during embryogenesis. Previous studies in mice suggest that TET1 is required for pluripotency and maintenance of embryonic stem cells by managing their epigenetic marks, specifically DNA methylation. This raises the possibility that TET1 is capable of establishing distinct epigenetic marks during embryo development, thus regulating pluripotency-related genes. However, this has not been demonstrated in any species. Previously we have demonstrated that the level of TET1 (mRNA and protein) was high in porcine blastocysts. In this study, we generated TET1 knockout embryos and analysed expression patterns of pluripotency-related genes in blastocysts to study the role of TET1 in maintaining pluripotency during porcine embryo development. The CRISPR/Cas9 system was applied to disrupt the TET1 gene during embryogenesis. Three single-guide RNAs (sgRNAs) were designed based on our previous cloning of the TET1 gene. In vitro-synthesised Cas9 mRNA (20 ng µL−1) and sgRNAs (10 ng µL−1 each) were injected into the cytoplasm of zygotes after IVF. A total of 605 zygotes were used for microinjection and subsequently 54 blastocysts were formed. As a control, 89 IVF blastocysts were developed from 240 embryos. Nine to ten blastocysts per group were collected on Day 7 to analyse gene expression patterns of TET family and pluripotency-related genes using quantitative RT-PCR. Three biological and 3 experimental replications were used. Differences in the gene expression were evaluated by ANOVA. As expected, there was a 2-fold decrease in the transcript level of TET1 in TET1-knockout blastocysts compared with that in control IVF blastocysts (P