Expression of porcine growth hormone gene in transgenic rabbits as reported by green fluorescent protein

Genetically modified rabbit models for cardiovascular medicine

Genetically modified (GM) rabbits are outstanding animal models for studying human genetic and acquired diseases. As such, GM rabbits that express human genes have been extensively used as models of cardiovascular disease. Rabbits are genetically modified via prokaryotic microinjection. Through this process, genes are randomly integrated into the rabbit genome. Moreover, gene targeting in embryonic stem (ES) cells is a powerful tool for understanding gene function. However, rabbits lack stable ES cell lines. Therefore, ES-dependent gene targeting is not possible in rabbits. Nevertheless, the RNA interference technique is rapidly becoming a useful experimental tool that enables researchers to knock down specific gene expression, which leads to the genetic modification of rabbits. Recently, with the emergence of new genetic technology, such as zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), clustered regularly interspaced short palindromic repeats (CRISPR), and CRISPR-associated protein 9 (CRISPR/Cas9), major breakthroughs have been made in rabbit gene targeting. Using these novel genetic techniques, researchers have successfully modified knockout (KO) rabbit models. In this paper, we aimed to review the recent advances in GM technology in rabbits and highlight their application as models for cardiovascular medicine.

Genetic Manipulation of the Equine Oocyte and Embryo

As standard in vitro fertilization is not a viable technique in horses yet, many different techniques have been used to create equine embryos for research purposes. One such method is parthenogenesis in which an oocyte is induced to mature into an embryo-like state without the introduction of a spermatozoon, and thus they are not considered true embryos. Another method is somatic cell nuclear transfer (SCNT), in which a somatic cell nucleus from an extant horse is inserted into an enucleated oocyte, creating a genetic clone of the donor horse. Due to limited availability of equine oocytes in the United States, researchers have investigated the potential for combining equine somatic cell nuclei with oocytes from other species to make embryos for research purposes, which has not been successful to date. There has also been a rising interest in producing transgenic animals using sperm exposed to exogenous DNA. The successful creation of transgenic equine blastocysts shows the promise of sperm mediated gene transfer (SMGT), but this method is not ideal for other applications, like gene therapy, because it cannot be used to induce targeted mutations. That is why technologies like CRISPR/Cas9 are vital. In this review, we argue that parthenogenesis, SCNT, and interspecies SCNT can be considered genetic manipulation strategies as they create embryos that are genetically identical to their parent cell. Here, we describe how these methods are performed and their applications and we also describe the few methods that have been used to directly modify equine embryos: SMGT and CRISPR/Cas9.

Altered Response to Total Body Irradiation of C57BL/6-Tg (CAG-EGFP) Mice

Application of green fluorescent protein (GFP) in a variety of biosystems as a unique bioindicator or biomarker has revolutionized biological research and made groundbreaking achievements, while increasing evidence has shown alterations in biological properties and physiological functions of the cells and animals overexpressing transgenic GFP. In this work, response to total body irradiation (TBI) was comparatively studied in GFP transgenic C57BL/6-Tg (CAG-EGFP) mice and C57BL/6 N wild type mice. It was demonstrated that GFP transgenic mice were more sensitive to radiation-induced bone marrow death, and no adaptive response could be induced. In the nucleated bone marrow cells of GFP transgenic mice exposed to a middle dose, there was a significant increase in both the percentage of cells expressing pro-apoptotic gene Bax and apoptotic cell death. While in wild type cells, lower expression of pro-apoptotic gene Bax and higher expression of anti-apoptotic gene Bcl-2, and significant lower induction of apoptosis were observed compared to GFP transgenic cells. Results suggest that presence of GFP could alter response to TBI at whole body, cellular and molecular levels in mice. These findings indicate that there could be a major influence on the interpretation of the results obtained in GFP transgenic mice.

Generation of transgenic cynomolgus monkeys that express green fluorescent protein throughout the whole body

Nonhuman primates are valuable for human disease modelling, because rodents poorly recapitulate some human diseases such as Parkinson’s disease and Alzheimer’s disease amongst others. Here, we report for the first time, the generation of green fluorescent protein (GFP) transgenic cynomolgus monkeys by lentivirus infection. Our data show that the use of a human cytomegalovirus immediate-early enhancer and chicken beta actin promoter (CAG) directed the ubiquitous expression of the transgene in cynomolgus monkeys. We also found that injection into mature oocytes before fertilization achieved homogenous expression of GFP in each tissue, including the amnion, and fibroblasts, whereas injection into fertilized oocytes generated a transgenic cynomolgus monkey with mosaic GFP expression. Thus, the injection timing was important to create transgenic cynomolgus monkeys that expressed GFP homogenously in each of the various tissues. The strategy established in this work will be useful for the generation of transgenic cynomolgus monkeys for transplantation studies as well as biomedical research.

A novel GFP nude rat model to investigate tumor-stroma interactions

BACKGROUD

A key strategy for the study of the tumor microenvironment is to implant human tumor cells in an immunodeficient rodent strain ubiquitously expressing a fluorescent marker. Here, a novel nude rat expressing a green fluorescent protein (GFP) transgene was established and engrafted with primary human tumor tissue in order to separate tumor from stromal cell populations for subsequent molecular analysis.

METHODS

SD-TG (GFP) 2BalRrrc transgenic rats were crossed with HsdHan™: rnu/rnu Rowett nude rats to develop a GFP expressing immunocompromised rat. PCR and flow cytometry were used to follow the GFP genotype and phenotype in newborns. After three to four generations, animals were implanted with 4 T1 dsRed murine breast cancer cells or primary human glioblastoma (GBM) biopsies to generate xenografts for subsequent separation by fluorescence-activated cell sorting (FACS).

RESULTS

Fluorecence microscopy and reverse transcription-PCR demonstrated that GFP, under the control of the human ubiquitin C promoter, was stably maintained and expressed in diverse organs over several generations. Immunophenotyping of blood samples by flow cytometry confirmed that the immunodeficient features of the parental rat strain, HsdHan™: rnu/rnu, were retained in the GFP nude rat. Both the murine cell line and human GBM biopsies engrafted, and stromal cell populations were isolated from dissociated xenografts by FACS to > 95% purity.

CONCLUSIONS

A GFP transgene was stably introduced into a nude rat background in which human and murine cancer cells successfully engrafted. This animal strain provides a novel in vivo system for detailed cellular and molecular characterization of tumor-stroma interactions.

Methods for sperm-mediated gene transfer

The transgenic technologies represent potent biotechnological tools that allow the generation of genetically modified animals useful for basic research and for biomedical, veterinary, and agricultural applications. Among transgenic techniques, we describe here the sperm-mediated gene transfer methods that is gene transfer based on the spontaneous ability of sperm cells to bind and internalize exogenous DNA and to carry it to oocyte during fertilization, producing genetically modified animals with high efficiency.

Production of transgenic pigs mediated by pseudotyped lentivirus and sperm

Sperm-mediated gene transfer can be a very efficient method to produce transgenic pigs, however, the results from different laboratories had not been widely repeated. Genomic integration of transgene by injection of pseudotyped lentivirus to the perivitelline space has been proved to be a reliable route to generate transgenic animals. To test whether transgene in the lentivirus can be delivered by sperm, we studied incubation of pseudotyped lentiviruses and sperm before insemination. After incubation with pig spermatozoa, 62±3 lentiviral particles were detected per 100 sperm cells using quantitative real-time RT-PCR. The association of lentivirus with sperm was further confirmed by electron microscopy. The sperm incubated with lentiviral particles were artificially inseminated into pigs. Of the 59 piglets born from inseminated 5 sows, 6 piglets (10.17%) carried the transgene based on the PCR identification. Foreign gene and EGFP was successfully detected in ear tissue biopsies from two PCR-positive pigs, revealed via in situ hybridization and immunohistochemistry. Offspring of one PCR-positive boar with normal sows showed PCR-positive. Two PCR-positive founders and offsprings of PCR-positive boar were further identified by Southern-blot analysis, out of which the two founders and two offsprings were positive in Southern blotting, strongly indicating integration of foreign gene into genome. The results indicate that incubation of sperm with pseudotyped lentiviruses can incorporated with sperm-mediated gene transfer to produce transgenic pigs with improved efficiency.

GFP-transgenic animals for in vivo imaging: rats, rabbits, and pigs

Specifically, gene-encoded biological probes serve as stable and high-performance tools to visualize cellular fate in living animals. The rat, as with the mouse, has offered important animal models for biology and medical research, and has provided a wealth of physiological and pharmacological data. The larger-body animals, in comparison to the mouse have allowed the application of various physiological and surgical manipulations that may prove to have biological significance. We have further extended the techniques of genetic engineering to rats, rabbits, and pigs, and have created corresponding GFP-transgenic animals. The GFP-positive organs of these animals provide valuable sensors in preclinical settings for cell therapy and transplantation studies. In this chapter, we highlight expression profiles in these animal resources and describe examples of preclinical applications.

Sperm and testis mediated DNA transfer as a means of gene therapy

Assisted reproductive technologies (ART) have revolutionized the treatment of infertility. However, many types of infertility may still not be addressable by ART. With recent successes in identifying many of the genetic factors responsible for male infertility and the future prospect of whole individual human genome sequencing to identify disease causing genes, the possible use of gene therapy for treating infertility deserves serious consideration. Gene therapy in the sperm and testis offers both opportunities and obstacles. The opportunities stem from the fact that numerous different approaches have been developed for introducing transgenes into the sperm and testis, mainly because of the interest in using sperm mediated gene transfer and testis mediated gene transfer as ways to generate transgenic animals. The obstacles arise from the fact that it may be very difficult to carry out gene therapy of the testis and sperm without also affecting the germline. Here we consider new developments in both sperm and testis mediated gene transfer, including the use of viral vectors, as well as the technical and ethical challenges facing those who would seek to use these approaches for gene therapy as a way to treat male infertility.

Production of transgenic rabbit embryos through intracytoplasmic sperm injection

The objective of this study was to test if intracytoplasmic sperm injection (ICSI)-mediated gene transfer was an effective method in the production of transgenic rabbit embryos. Rabbit sperm diluted in different media with various pH were treated by freezing without cryoprotectant, and their ability for DNA uptake was determined. In these experiments using production of transgenic rabbit embryos by ICSI, exogenous genes at three concentrations and of two conformation types were used. The rate of DNA association to the sperm seen by rhodamine-tagged DNA encoding green fluorescent protein (GFP) was 90.0%, 92.7%, 91.0%, 91.7%, and 92.3%, respectively in TCM199, DM, DPBS, CZB, and HCZB media. The DNA attachment to sperm was not affected by media pH within the range of 5.4-9.4 (p > 0.05). Expression of GFP first occurred at the 2-cell stage and continued to blastocyst formation. DNA concentration (between 5, 10, and 20 ng/μl) or conformation (linear and circular) had no effect on the production rate of transgenic embryos. These results indicated that genetically modified rabbit blastocysts can be efficiently produced by ICSI technique.

Quality of transgenic rabbit embryos with different EGFP gene constructs

The aim of this study was to compare the quality of rabbit transgenic embryos obtained upon microinjection of gene constructs containing different promoters and green fluorescent proteins (CMVIE–EGFP, PGK–EGFP and CMVIE–hrGFP). Developmental rate, total cell number in hatching blastocyst stage, number of apoptotic cells, diameter of embryos, transgene integration and transgenic mosaicism were investigated.

The rate of rabbit embryos microinjected with the different gene constructs developed up to morula stage was significantly lower (p < 0.05) than that of intact (non-microinjected) rabbit embryos (66–74vs. 98%). The highest efficiency of transgene integration (15%) was found when the CMVIE–EGFP (DrdI) gene construct was used, however a significantly higher transgenic mosaicism (60%) was found in rabbit embryos using this gene. The lowest cell number was counted in rabbit transgenic embryos with CMVIE–rhGFP linearized by ScaI (115.0 ± 8.20), the highest cell number (134.0 ± 35.00) was detected in rabbit transgenic embryos carrying PGK–EGFP (Not I) gene. The highest number of apoptotic cells (2.6 ± 0.33) was recorded in rabbit transgenic embryos with the integrated CMVIE–EGFP (ClaI) transgene.

Based on these results a more suitable gene marker for rabbit transgenic embryos production and selection is the CMVIE–EGFP (ClaI) gene construct. Prior to using microinjected embryos (for embryo transfer, vitrification or ESC isolation) it is necessary to pre-select microinjected embryos with evident transgenic mosaicism.

In vivoGene Transfer into Testis and Sperm: Developments and Future Application

Despite significant advances in the treatment of infertility via assisted reproductive technology (ART), the underlying causes of idiopathic male infertility still remain unclear. Accumulating evidence suggests that disorders associated with testicular gene expression may play an important role in male infertility. To be able to fully study the molecular mechanisms underlying spermatogenesis and fertilization, it is necessary to manipulate gene expression in male germ cells. Since there is still no reliable method of recapitulating spermatogenesis culture, the development of alternative transgenic approaches is paramount in the study of gene function in testis and sperm. Established methods of creating transgenic animals rely heavily upon injection of DNA into the pronucleus or the injection of transfected embryonic stem cells into blastocysts to form chimeras. Despite the success of these two approaches for making transgenic and knockout animals, concerns remain over costs and the efficiency of transgene integration. Consequently, efforts are in hand to evaluate alternative methodologies. At present, there is much interest in developing approaches that utilize spermatozoa as vectors for gene transfer. These approaches, including testis mediated gene transfer (TMGT) and sperm mediated gene transfer (SMGT), have great potential as tools for infertility research and in the creation of transgenic animals. The aim of this short review is to briefly describe developments in this field and discuss how these gene transfer methods might be used effectively in future research and clinical arenas.

Full-term development of rabbit embryos produced by ICSI with sperm frozen in liquid nitrogen without cryoprotectants

The aim of the present study was to establish the technology of intracytoplasmic sperm injection (ICSI) in rabbit by using the sperm frozen without cryoprotectants. Observation under an electron microscope revealed that the rabbit spermatozoa frozen without cryoprotectants had severe damage especially in the plasma membrane and junction between head and tail. However, after being injected into the oocytes, the sperm frozen without cryoprotectants retained the capability of supporting the cleavage and development of the ICSI oocytes, with no significant difference from that of fresh sperm, although the development of ICSI embryos derived from either frozen sperm or fresh sperm is much lower than that of in vivo-fertilized zygotes. When additional artificial activation was applied following ICSI, the rates of cleavage and blastocyst formation of ICSI oocytes were significantly increased when compared with the oocytes without additional activation. Yet, the cell numbers in blastocysts were not significantly different between the activation and non-activation group. After embryo transfer, four offspring were obtained from the oocytes microinjected with the sperm frozen without cryoprotectants. The technology established by this study may facilitate exploring the ICSI-based transgenic method in rabbit and broaden the application of ICSI technique in related field.

Sperm-mediated 'reverse' gene transfer: a role of reverse transcriptase in the generation of new genetic information

Sperm-mediated gene transfer (SMGT) is a procedure through which new genetic traits are introduced in animals by exploiting the ability of spermatozoa to take up exogenous DNA molecules and deliver them to oocytes at fertilization. The interaction of exogenous DNA with sperm cells is a regulated process mediated by specific factors; among those, a reverse transcriptase (RT) activity plays a central role in SMGT. 'Retro-genes' are generated either through reverse transcription of exogenous RNA internalized in spermatozoa, or through sequential transcription, splicing and reverse transcription of exogenous DNA. The resulting retro-genes are delivered to oocytes and transmitted to embryos and born animals as low-copy, transcriptionally competent, extrachromosomal structures capable of determining new phenotypic traits. Retro-genes can be further transmitted through sexual reproduction from founders to their F1 progeny: new genetic and phenotypic features, unlinked to chromosomes, can thus be generated and inherited in a non-Mendelian ratio. We have called this phenomenon sperm-mediated 'reverse' gene transfer (SMRGT). Thus, a RT-mediated machinery operates in sperm cells and is responsible for the genesis and non-Mendelian propagation of new genetic information. The features of RT-generated traits elicited in SMRGT resemble those characterized in recent studies of RNA-mediated inheritance of extra-genomic information.

Establishment and characterization of CAG/EGFP transgenic rabbit line

Cell marking is a very important procedure for identifying donor cells after cell and/or organ transplantation in vivo. Transgenic animals expressing marker proteins such as enhanced green fluorescent protein (EGFP) in their tissues are a powerful tool for research in fields of tissue engineering and regenerative medicine. The purpose of this study was to establish transgenic rabbit lines that ubiquitously express EGFP under the control of the cytomegalovirus immediate early enhancer/beta-actin promoter (CAG) to provide a fluorescent transgenic animal as a bioresource. We microinjected the EGFP expression vector into 945 rabbit eggs and 4 independent transgenic candidate pups were obtained. Two of them died before sexual maturation and one was infertile. One transgenic male candidate founder rabbit was obtained and could be bred by artificial insemination. The rabbit transmitted the transgene in a Mendelian manner. Using fluorescence in situ hybridization analysis, we detected the transgene at 7q11 on chromosome 7 as a large centromeric region in two F1 offspring (one female and one male). Eventually, one transgenic line was established. Ubiquitous EGFP fluorescence was confirmed in all examined organs. There were no gender-related differences in fluorescence. The established CAG/EGFP transgenic rabbit will be an important bioresource and a useful tool for various studies in tissue engineering and regenerative medicine.

Increased transgene integration efficiency upon microinjection of DNA into both pronuclei of rabbit embryos

Transgenic rabbits provide a useful biological model for the study of the regulation of mammalian genes. However, transgene integration efficiency has generally been low. Here we present a first attempt to increase the integration rate of exogenous DNA into the rabbit genome, using a double pronuclei microinjection method. Pronuclear stage rabbit embryos were recovered from superovulated NZW females, 19-20 h after hCG injection. About 5 microg/mL of exogenous DNA solution was microinjected either into one pronucleus (single microinjection, SM) or into both pronuclei (double microinjected, DM). The transgene consisted of a 2.5 kb murine whey acidic protein promoter (mWAP), 7.2 kb cDNA of the human clotting factor VIII (hFVIII), and 4.6 kb that of 3' flanking sequences of the mWAP gene. The in vitro survival of DM embryos to the blastocyst stage was lower than that of SM embryos (68 vs. 89%). Similar results were obtained using EGFP as a control gene construct. However, there was no difference in the percentage of embryos that developed into live offspring using DM (25%) vs. SM (26%). The integration frequency of mWAP-hFVIII into the genome of transgenic rabbits was 3.3% (1/30) upon SM and 8.1% (4/49) at DM (p < 0.05). All founders transmitted the transgene to their offspring in a Mendelian fashion. The SM founder female secreted 87.4 microg/mL rhFVIII in milk, with an activity of 0.594 IU/mL. The DM founder female produced 118 microg/mL rhFVIII, with activity values of 18 IU/ mL. This is the first report of transgenic rabbit production using a double microinjection technique. Our preliminary results suggest that this method can increase the efficiency of production of transgenic rabbit founders, giving a higher integration rate than single microinjection.

Use of transgenic animals to improve human health and animal production

Contents Transgenic animals are more widely used for various purposes. Applications of animal transgenesis may be divided into three major categories: (i) to obtain information on gene function and regulation as well as on human diseases, (ii) to obtain high value products (recombinant pharmaceutical proteins and xeno-organs for humans) to be used for human therapy, and (iii) to improve animal products for human consumption. All these applications are directly or not related to human health. Animal transgenesis started in 1980. Important improvement of the methods has been made and are still being achieved to reduce cost as well as killing of animals and to improve the relevance of the models. This includes gene transfer and design of reliable vectors for transgene expression. This review describes the state of the art of animal transgenesis from a technical point of view. It also reports some of the applications in the medical field based on the use of transgenic animal models. The advance in the generation of pigs to be used as the source of organs for patients and in the preparation of pharmaceutical proteins from milk and other possible biological fluids from transgenic animals is described. The projects in course aiming at improving animal production by transgenesis are also depicted. Some the specific biosafety and bioethical problems raised by the different applications of transgenesis, including consumption of transgenic animal products are discussed.

Sperm-mediated gene transfer: applications and implications

Recent developments in studies of sperm-mediated gene transfer (SMGT) now provide solid ground for the notion that sperm cells can act as vectors for exogenous genetic sequences. A substantive body of evidence indicates that SMGT is potentially useable in animal transgenesis, but also suggests that the final fate of the exogenous sequences transferred by sperm is not always predictable. The analysis of SMGT-derived offspring has shown the existence of integrated foreign sequences in some cases, while in others stable modifications of the genome are difficult to detect. The appearance of SMGT-derived modified offspring on the one hand and, on the other hand, the rarity of actual modification of the genome, suggest inheritance as extrachromosomal structures. Several specific factors have been identified that mediate distinct steps in SMGT. Among those, a prominent role is played by an endogenous reverse transcriptase of retrotransposon origin. Mature spermatozoa are naturally protected against the intrusion of foreign nucleic acid molecules; however, particular environmental conditions, such as those occurring during human assisted reproduction, can abolish this protection. The possibility that sperm cells under these conditions carry genetic sequences affecting the integrity or identity of the host genome should be critically considered. These considerations further suggest the possibility that SMGT events may occasionally take place in nature, with profound implications for evolutionary processes.

Association of Rabbit Sperm Cells with Exogenous DNA

The ability of rabbit spermatozoa to bind exogenous DNA during sperm-mediated gene transfer (SMGT) was tested in our study. Fresh collected semen, or fully capacitated sperm cells, was co-cultured with plasmid DNA labeled with tetramethyrodamine-6-dUTP. Fluorescent spermatozoa were counted before and after DNaseI treatment. Results showed that fluorescent-labeled plasmid DNA could be taken up by capacitated rabbit sperm cells. 66% spermatozoa carried exogenous DNA in the presence of lipofectin. Bovine serum albumin could block this process effectively. Associated DNA was mainly located in the posterior area of the sperm head. In order to verify whether exogenous DNA was carried into the embryo and expressed in the offspring, further SMGT experiments were carried out using the pHM-CR plasmid which contains LacZ and Neomycin genes. beta-galactosidase was expressed in different stages of embryo development and in the tissues of young rabbit as detected by using X-gal staining. Large portion of embryos survived under the selection pressure in G418 containing medium, after SMGT. Transgene integration was further verified by PCR analysis. These results confirmed the ability of rabbit sperm cells to carry transgene into the embryo during in vitro fertilization.

Transgenic rabbits as therapeutic protein bioreactors and human disease models

Genetically modified laboratory animals provide a powerful approach for studying gene expression and regulation and allow one to directly examine structure-function and cause-and-effect relationships in pathophysiological processes. Today, transgenic mice are available as a research tool in almost every research institution. On the other hand, the development of a relatively large mammalian transgenic model, transgenic rabbits, has provided unprecedented opportunities for investigators to study the mechanisms of human diseases and has also provided an alternative way to produce therapeutic proteins to treat human diseases. Transgenic rabbits expressing human genes have been used as a model for cardiovascular disease, AIDS, and cancer research. The recombinant proteins can be produced from the milk of transgenic rabbits not only at lower cost but also on a relatively large scale. One of the most promising and attractive recombinant proteins derived from transgenic rabbit milk, human alpha-glucosidase, has been successfully used to treat the patients who are genetically deficient in this enzyme. Although the pronuclear microinjection is still the major and most popular method for the creation of transgenic rabbits, recent progress in gene targeting and animal cloning has opened new avenues that should make it possible to produce transgenic rabbits by somatic cell nuclear transfer in the future. Based on a computer-assisted search of the studies of transgenic rabbits published in the English literature here, we introduce to the reader the achievements made thus far with transgenic rabbits, with emphasis on the application of these rabbits as human disease models and live bioreactors for producing human therapeutic proteins and on the recent progress in cloned rabbits.