Expression of two transgenes in in vitro matured and fertilized bovine zygotes after DNA microinjection

One-step Multiplex Transgenesis via Sleeping Beauty Transposition in Cattle

Genetically modified cattle are important for developing new biomedical models and for an improved understanding of the pathophysiology of zoonotic diseases. However, genome editing and genetic engineering based on somatic cell nuclear transfer suffer from a low overall efficiency. Here, we established a highly efficient one-step multiplex gene transfer system into the bovine genome.

Optimized production of transgenic buffalo embryos and offspring by cytoplasmic zygote injection

Background

Cytoplasmic injection of exogenous DNA into zygotes is a promising technique to generate transgenic livestock. However, it is still relatively inefficient and has not yet been demonstrated to work in buffalo. We sought to improve two key technical parameters of the procedure, namely i) how much linear DNA to inject and ii) when to inject it. For this, we introduced a constitutively expressed enhanced green fluorescent protein (EGFP) plasmid into buffalo zygotes.

Results

First, we found that the proportion of EGFP-expressing blastocysts derived from zygotes injected with 20 or 50 ng/μL DNA was significantly higher than from those injected with 5 μg/mL. However, 50 ng/μL exogenous DNA compromised blastocyst development compared to non-injected IVF controls. Therefore the highest net yield of EGFP-positive blastocysts was achieved at 20 ng/μL DNA. Second, zygotes injected early (7–8 h post-insemination [hpi]) developed better than those injected at mid (12–13 hpi) or late (18–19 hpi) time points. Blastocysts derived from early injections were also more frequently EGFP-positive. As a consequence, the net yield of EGFP-expressing blastocysts was more than doubled using early vs late injections (16.4 % vs 7.7 %). With respect to blastocyst quality, we found no significant difference in cell numbers of EGFP-positive blastocysts vs non-injected blastocysts. Following embryo transfer of six EGFP-positive blastocysts into four recipient animals, two viable buffalo calves were born. Biopsied ear tissues from both buffalo calves were analyzed for transgene presence and expression by Southern blot, PCR and confocal laser scanning microscopy, respectively. This confirmed that both calves were transgenic.

Conclusions

Our cytoplasmic injection protocol improved generation of transgenic embryos and resulted in the first transgenic buffalo calves produced by this method.

Efficient transgene expression in IVF and parthenogenetic bovine embryos by intracytoplasmic injection of DNA-liposome complexes

Transgenic animals constitute an important tool with many biotechnological applications. Although there have been advances in this field, we propose a novel method that may greatly increase the efficiency of transgenic animal production and thereby its application. This new technique consists of intracytoplasmic injection of liposomes, in bovine oocytes and zygotes, to introduce exogenous DNA. In the first experiment, we evaluated embryo development and EGFP expression in In Vitro Fertilization (IVF) embryos injected with different concentrations of exogenous DNA-liposome complexes (0.5, 5, 50, 500 ng pCX-EGFP/μl). The highest EGFP-embryos rates were obtained using 500 ng pCX-EGFP/μl. In the second experiment, we evaluated embryo development and EGFP expression following the injection of DNA-liposome complexes into pre-fertilized oocytes and presumptive zygotes, 16 and 24 h post-fertilization. Approximately 70% of the cleaved embryos and 50% of the blastocysts expressed EGFP, when egfp-liposome was injected 16 h post-fertilization. The percentages of positive embryos for the 24-h post-fertilization and pre-fertilization groups were 30.1 and 6.3, respectively. Blastocysts that developed from injected zygotes were analysed by PCR, confirming the presence of transgene in all embryos. Finally, we examined the embryo development and EGFP expression of parthenogenetic embryos that resulted from the injection of egfp-liposome complexes into pre-activated oocytes, and 3 and 11 h post-activated oocytes. The group with the highest expression rate (48.4%) was the one injected 3 h post-activation. In summary, this study reports the efficient, reproducible and fast production of IVF and parthenogenetic embryos expressing EGFP, by the intracytoplasmic injection of liposomes to introduce the foreign DNA.

Pronuclear synchronization and nuclear morphology of mature and in vitro matured oocytes in the rat: an ultrastructural study

The objective of this study was to evaluate synchronous and asynchronous pronucleus (PN) formation and the related patterns of juxtapositional nucleolus (n) formation in immature (prophase I [PI] and metaphase I [MI]) and mature (metaphase II [MII]) oocytes after fertilization, both ultrastructurally and at the level of light microscope. A single dose of 15 IU gonadotrophin was injected subcutaneously to twenty four 26-wk-old, female Wistar rats to induce ovulation. Human chorionic gonadotrophin (4 IU) was administered 40 h later, and after 4-6 h the ovaries were dissected, and the oocytes were aspirated. A total of 214 rat oocytes were classified according to a maturation index as follows: group I, 80 PI oocytes; group II, 50 MI oocytes; and group III, 84 MII oocytes. Immature oocytes were in vitro matured for 18-36 h. Spermatozoa were acquired by microepididymal sperm aspiration and processed using swim-up technique. Intracytoplasmic sperm injection was performed on mature oocytes after 2 h of incubation and on in vitro matured (IVM) oocytes 4 h after maturation. Pronuclear synchronization [both pronucleases (PNs) centrally located, equal sized, with equal numbers and sizes of juxtapositional nucleoli (Nn)] was observed in fertilized oocytes. Asynchronous PN formation (diversity between male and female PNs, related to dimensions, localization, and the number of Nn) in groups I, II, and III was found in 75, 86, and 47% of preembryos, respectively. There was a significant difference of synchronous pronuclear formation between mature and IVM oocytes (P < 0.05). In IVM oocytes, asynchronous PN formation is high, and juxtapositional pronucleolar patterns are observed to be low by transmission electron microscope (TEM).

Transgenic animals: current and alternative strategies

Transgenic animal technology is one of the most fascinating technologies developed in the last two decades. It allows us to address questions in life sciences that no other methods have achieved. The impact on biomedical and biological research, as well as commercial interests are overwhelming. The questions accompanying this fast growing technology and its diversified applications attract the attention from a variety of entities. Still, one of the most fundamental problems remaining is the search for an efficient and reliable gene delivery system for creating transgenic animals. The traditional method of pronuclear microinjection has displayed great variability in success among species. While an acceptable efficiency in the production of transgenic mice has been attained, the relative low efficiency (<1%) in creating transgenic livestock has become one of the barriers for its application. In the past decades, improvements in producing transgenic livestock have made a slow progression, however, the recent advancement in cloning technology and the ability to create transgenic livestock in a highly efficient manner, have opened the gate to a new era in transgenic technology. Discoveries of new gene delivery systems have created an enthusiastic atmosphere that has made this technology so unique. This review focuses on gene delivery strategies as well as various approaches that may assist the advancement of transgenic efficiency in large animals.

Expression pattern of Oct-4 in preimplantation embryos of different species

POU transcription factors are involved in transcriptional regulation during early embryonic development and cell differentiation. Oct-4, a member of this family, has been shown to be under strict regulation during murine development. The expression of Oct-4 correlates with the undifferentiated cell phenotype of the mouse preimplantation embryo. In this study, expression of a gene construct consisting of selected parts of the region upstream from the murine Oct-4 gene as promoter/enhancer, enhanced green fluorescent protein (EGFP) as reporter and the five exons of the murine Oct-4 gene (GOF18-delta PE EGFP) was evaluated in murine, porcine, and bovine preimplantation embryos. For comparison, expression of the endogenous Oct-4 gene was also analyzed in all three species by immunocytochemistry. The transgene construct was microinjected into zygotes cultured in vitro to various developmental stages. The EGFP fluorescence was visualized in developing embryos by excitation with blue light at different days following microinjection and showed similar expression patterns in all three species. Most embryos displayed a mosaic pattern of transgene expression. The EGFP fluorescence was not restricted to the inner cell mass (ICM) but was also seen in trophoblastic cells. An affinity-purified polyclonal antibody specific to Oct-4 was used for immunocytochemical analysis of in vivo- and in vitro-derived bovine and porcine blastocysts and also of in vivo-derived murine blastocysts. In the in vivo-derived murine embryos, Oct-4 protein was detectable in the ICM but not the trophectoderm, whereas in porcine and bovine blastocysts, derived in vivo or in vitro, Oct-4 protein was detected in both the ICM and the trophectoderm. Thus, in the two large animal species, Oct-4 expression from the endogenous gene was clearly not restricted to the pluripotent cells of the early embryo. These results show that Oct-4 regulation differs between these species and that the presence of Oct-4 protein may not be sufficient for selection of undifferentiated cell lines in domestic animals.

Efficient selection of transgenic mouse embryos using EGFP as a marker gene

We have established a reliable method that uses the EGFP (Enhanced Green Fluorescent Protein) gene as a marker for selecting transgenic embryos from preimplantation embryos. Embryos that were subjected to the pronuclear microinjection of the CMV/beta-actin/EGFP fusion gene were cultured in vitro until they developed into the morulae- or blastocyst-stage. The expression of EGFP was easily observed by a fluorescent microscopy. There appeared to be no damage to the in vivo developmental ability of the embryos in response to the EGFP excitation light, which utilized an IB filter for a period of 30 min. Modified PCR analysis using Dpn I and Bal 31 digestion of the embryonic DNA showed that all of the embryos expressing EGFP in all their cells were transgenic, while more than half with mosaic expression of EGFP were not transgenic. Approximately 77% of pups born from the embryos that uniformly expressed the EGFP gene were transgenic, while 21.4% of pups from the embryos with mosaic expression were transgenics. The results showed that the use of EGFP as a marker is very useful and reliable for selecting transgenic embryos, and that it is important to transfer the embryos expressing EGFP in all their cells to obtain truly transgenic animals.

Bovine follicular dynamics, oocyte recovery, and development of oocytes microinjected with a green fluorescent protein construct

The present study was carried out to 1) evaluate the viability of in vitro fertilized zygotes after microinjection of DNA, 2) assess the influence of oocyte quality upon the development rate of embryos when injected with DNA, and 3) determine the integration frequency of green fluorescent protein DNA into microinjected embryos. Oocytes were aspirated from ovaries of nine nonlactating Holsteins and were categorized into grades A, B, C, and D. At 16 h after in vitro fertilization, approximately half of the pronuclear stage presumptive zygotes were classified as having 1 pronucleus or 2 pronuclei, and they were microinjected with DNA constructs. A potential predictor of DNA integration frequency at d 10 was assessment of the incidence of green fluorescing embryos. The proportion of cleaved embryos that developed to morulae or blastocysts was not different between groups with 1 pronucleus injected (45%), 1 pronucleus uninjected (64%), or 2 pronuclei injected (49%). However, the development of morulae or blastocysts was higher in the group with 2 pronuclei uninjected (69%). The overall developmental score of green fluorescent protein-positive embryos was higher for grade A oocytes (1.3 +/- 0.1) than for grade B (0.8 +/- 0.1), C (0.6 +/- 0.1), or D (0.3 +/- 0.1) oocytes. The results show that production of transgenic bovine blastocysts can occur from the microinjection of a presumptive zygote having only one visible pronucleus. Initial oocyte quality is an important factor in selection of oocytes suitable for microinjection of DNA and for preimplantation development to produce bovine transgenic embryos.

Timing of DNA integration, transgenic mosaicism, and pronuclear microinjection

Selection of transgenic embryos prior to embryo transfer is a means to increase the efficiency of transgenic livestock production. Among transgenic reporters, cytoplasmic expression of green fluorescent protein (GFP) has features that make it ideal for transgenic embryo selection. The primary objective of this study was to assess cytoplasmic expression of a specially designed GFP reporter as a tool for transgenic bovine embryo selection. A second objective was to evaluate this reporter for studying transgenic mosaicism related to timing of integration of pronuclear microinjected DNA. Transgenic embryos produced by pronuclear injection showed a discrete pattern of GFP expression with clusters at 25, 50, and 100% of blastomeres expressing GFP. This pattern of mosaicism is interpreted to indicate that the integration of microinjected DNA occurred, not only at the pronuclear stage, but also in the subsequent cell divisions. Among the GFP-positive transgenic embryos, only in 21% did all the blastomeres show the green fluorescence. Using the fraction of positive blastomeres within an embryo, the timing of integration of microinjected DNA was estimated. The frequency of nonmosaic embryos expressing GFP is consistent with published germline transmission success rates of transgenic cattle derived from pronuclear microinjected embryos. These results indicate the possible application of GFP as a marker of transgenic embryos and graphically illustrate underlying complexities in DNA integration in embryos subjected to pronuclear microinjection.

Prediction of transgene integration by noninvasive bioluminescent screening of microinjected bovine embryos

Transgenesis in domestic species, as a research tool and in biotechnological applications, has been limited by the expense of producing transgenic offspring by standard microinjection techniques. A major factor is the inefficiency of maintaining large numbers of recipient females, when a high percentage of these carry nontransgenic fetuses. There are two approaches to reduce this cost, the fusion of transfected fetal fibroblasts with enucleated oocytes, and the screening of microinjected embryos for transgene integration in blastocysts, prior to transfer. Here, we develop a luminescent screening system to select transgenic bovine embryos. A transgene with scaffold attachment regions flanking the murine HSP70.1 promoter linked to firefly luciferase cDNA, was microinjected into pronuclei of in vitro produced zygotes. At the blastocyst stage, the transgene was induced by heat shock (45 degrees C, 15 min) and 4-6 h later, luciferase expression was analyzed by photon counting imaging. Screened blastocysts were transferred to recipients and day 50 fetuses or calves were analyzed by PCR and Southern blot for transgene integration. When nonluminescent blastocysts were transferred, transgene integration was never observed. Of 13 fetuses derived from luminescent blastocysts, 3 contained integrated transgenes that were functional in all tissues examined. Image analysis of the signal emitted by positive blastocysts revealed that 9 nontransgenic fetuses were obtained from blastocysts that exhibited a localized luminescent signal. On the other hand, 3 of 4 fetuses derived from blastocysts that emitted light over more than 70% of their surface were transgenic. Thus, by selecting luminescent blastocysts on the basis of both signal intensity and distribution, the number of recipient females required to produce transgenic offspring can be greatly reduced. Using this technique it should also be possible to improve the efficiency of transgenesis by microinjection through studies in which vector design and integration conditions are examined at the blastocyst stage.

Quantitative analysis of luciferase activity of viral and hybrid promoters in bovine preimplantation embryos

This study was conducted to investigate quantitatively the luciferase activity of gene constructs with viral and hybrid enhancers and promoters in bovine preimplantation embryos by using firefly luciferase reporter genes. In Experiment I, to examine the stability of the luciferase, bioluminescence intensity of bovine embryos injected with the luciferase gene driven by the SV40 early promoter and enhancer (SVEluc) was measured with a luminometer at 2 days after microinjection. The results indicated that the bioluminescence could be analysed at any time within 30 min because the luciferase activity was constant during the measurement period from 5 to 30 min. In Experiment II, the luciferase expression of fertilized oocytes injected with four gene constructs (TKEluc, TK6WEluc, SVEluc, and Miwluc) was analysed by using a photon imaging system at 2 or 6 days following microinjection. The results from Experiment II indicated that the reporter gene governed by the Miw promoter (RSV LTR and chicken beta-actin promoter) was expressed more intensively in bovine morulae and blastocysts than three other gene constructs. In Experiment III, the effect of SV40 enhancer was investigated when fused downstream to the luciferase cDNA of the Miwluc vector. The results showed that SV40 enhancer further activated the luciferase activity of the Miw promoter in bovine preimplantation embryos. It was concluded, therefore, that the Miw promoter together with the SV40 enhancer would confer the strongest expression of the firefly luciferase reporter gene among the gene constructs tested in preimplantation bovine embryos.

Comparison of in vitro development and gene expression of in vivo- and IVM/IVF-derived porcine embryos after microinjection of foreign DNA

We compared the developmental ability and gene expression of in vivo- and IVM/IVF-derived porcine embryos following microinjection with SV40-LacZ. A total of 412 IVM/IVF-derived and 129 in vivo-collected zygotes was used to examine developmental ability and gene expression following DNA microinjection. When either DNA injected or noninjected zygotes were cultured for 4 d in NCSU 23 followed by 5 d in Eagle's minimal essential medium (EMEM), the percentages of zygotes developing to blastocysts and hatched blastocysts were higher (P < 0.05) compared with groups cultured in NCSU 23 alone. The percentages of injected embryos reaching the morula and blastocyst stages were significantly lower (P < 0.05) than that of noninjected control embryos whether in vivo or IVM/IVF derived. The percentages of morula and blastocyst stage embryos expressing the gene were higher in the in vivo-derived embryos than in IVM/IVF-derived embryos. A lower proportion of (67 to 77%) mosaicism was observed in the in vivo-derived embryos than in IVM/IVF (90 to 100%) derived embryos. The total cell number of blastocysts cultured in both NCSU 23 and EMEM media was significantly higher than that of blastocysts cultured continuously in NCSU 23. Our results suggest that this dual culture system enhanced embryo viability following microinjection of foreign DNA.

Reversible permeabilization of plasma membranes with an engineered switchable pore

By using an engineered, self-assembling, proteinaceous, 2-nm pore equipped with a metal-actuated switch, a technique to reversibly permeabilize the plasma membrane to small molecules (approximately 1000 Da) has been developed. We have demonstrated the dose-dependent permeabilization of fibroblasts by pores designed to be blocked and unblocked by the addition and removal of microM concentrations of Zn2+. Further, we have shown that the activity of the switch allows permeabilized cells to maintain viability and ultrastructural integrity following the unconstrained flux of small molecules. This ability to control the transmembrane influx and efflux of molecules and thereby vary the intracellular environment yet maintain cell viability will impact an array of biological and medical problems.