Production of transgenic rodents by the microinjection of cloned DNA into fertilized one-cell eggs

Genetically modified mice for research on human diseases: A triumph for Biotechnology or a work in progress?

Genetically modified mice are engineered as models for human diseases. These mouse models include inbred strains, mutants, gene knockouts, gene knockins, and ‘humanized’ mice. Each mouse model is engineered to mimic a specific disease based on a theory of the genetic basis of that disease. For example, to test the amyloid theory of Alzheimer’s disease, mice with amyloid precursor protein genes are engineered, and to test the tau theory, mice with tau genes are engineered. This paper discusses the importance of mouse models in basic research, drug discovery, and translational research, and examines the question of how to define the “best” mouse model of a disease. The critiques of animal models and the caveats in translating the results from animal models to the treatment of human disease are discussed. Since many diseases are heritable, multigenic, age-related and experience-dependent, resulting from multiple gene-gene and gene-environment interactions, it will be essential to develop mouse models that reflect these genetic, epigenetic and environmental factors from a developmental perspective. Such models would provide further insight into disease emergence, progression and the ability to model two-hit and multi-hit theories of disease. The summary examines the biotechnology for creating genetically modified mice which reflect these factors and how they might be used to discover new treatments for complex human diseases such as cancers, neurodevelopmental and neurodegenerative diseases.

PhiC31-based Site-Specific Transgenesis System for Production of Transgenic Bovine Embryos by Somatic Cell Nuclear Transfer and Intracytoplasmic Sperm Injection

OBJECTIVES

The Streptomyces phage phiC31 integrase offers a sequence-specific method of transgenesis with a robust long-term gene expression. PhiC31 has been successfully developed in a variety of tissues and organs for purpose of in vivo gene therapy. The objective of the present experiment was to evaluate PhiC31-based site-specific transgenesis system for production of transgenic bovine embryos by somatic cell nuclear transfer and intracytoplasmic sperm injection.

MATERIALS AND METHODS

In this experimental study, the application of phiC31 integrase system was evaluated for generating transgenic bovine embryos by somatic cell nuclear transfer (SCNT) and sperm mediated gene transfer (SMGT) approaches.

RESULTS

PhiC31 integrase mRNA and protein was produced in vitro and their functionality was confirmed. Seven phiC31 recognizable bovine pseudo attachment sites of phage (attP) sites were considered for evaluation of site specific recombination. The accuracy of these sites was validated in phic31 targeted bovine fibroblasts using polymerase chain reaction (PCR) and sequencing. The efficiency and site-specificity of phiC31 integrase system was also confirmed in generated transgenic bovine embryo which successfully obtained using SCNT and SMGT technique.

CONCLUSIONS

The results showed that both SMGT and SCNT-derived embryos were enhanced green fluorescent protein (EGFP) positive and phiC31 integrase could recombine the reporter gene in a site specific manner. These results demonstrate that attP site can be used as a proper location to conduct site directed transgenesis in both mammalian cells and embryos in phiC31 integrase system when even combinaed to SCNT and intracytoplasmic sperm injection (ICSI) method.

CRISPR/Cas-mediated genome editing in the rat via direct injection of one-cell embryos

Conventional embryonic stem cell (ESC)-based gene targeting, zinc-finger nuclease (ZFN) and transcription activator-like effector nuclease (TALEN) technologies are powerful strategies for the generation of genetically modified animals. Recently, the CRISPR/Cas system has emerged as an efficient and convenient alternative to these approaches. We have used the CRISPR/Cas system to generate rat strains that carry mutations in multiple genes through direct injection of RNAs into one-cell embryos, demonstrating the high efficiency of Cas9-mediated gene editing in rats for simultaneous generation of compound gene mutant models. Here we describe a stepwise procedure for the generation of knockout and knock-in rats. This protocol provides guidelines for the selection of genomic targets, synthesis of guide RNAs, design and construction of homologous recombination (HR) template vectors, embryo microinjection, and detection of mutations and insertions in founders or their progeny. The procedure from target design to identification of founders can take as little as 6 weeks, of which <10 d is actual hands-on working time.

Generation of Transgenic Rats Using Lentiviral Vectors

Transgenesis is a valuable tool with which to study different aspects of gene function in the context of the intact organism. During the last two decades a tremendous number of transgenic animals have been generated, and the continuous improvement of technology and the development of new systems have fostered their widespread application in biomedical research. Generally, transgenic animals are generated by introducing foreign DNA into fertilized oocytes, which can be achieved either by injecting recombinant DNA into the pronucleus or by transferring lentiviral particles into the perivitelline space. While mice remain the favored species in many laboratories, there are a number of applications where the use of rats is advantageous. One such research area is multiple sclerosis. Here, several experimental models are available that are closely mimicking the human disease, and it is possible to induce neuroinflammation by transferring pathogenic T cells which can then be studied by flow cytometry and 2-photon live imaging. Unlike for mice, the development of transgenic rats has encountered some hurdles in the past, e.g., due to a complicated reproductive biology and the frailty of the fertilized oocytes in vitro. In this chapter we provide a protocol describing how we manipulate single cell embryos in our lab in order to efficiently generate transgenic rats in a variety of different strains using lentiviral gene transfer.

The use of lentiviral vectors to obtain transgenic rats

Lentiviral vectors are now well recognized as good vehicles for gene delivery. This is because they can efficiently transduce both dividing and post-mitotic cells, and stably integrate into the host genome allowing for long-term expression of the transgene. Their potential utility for the generation of transgenic animals has been recognized as an attractive and promising alternative to the conventional DNA-microinjection method which lacks efficiency. The initial success of lentiviral transgenesis in mice considerably broadened its use in other species, in which classical transgenic techniques are difficult, such as in the rat.In this chapter, we describe detailed procedures for both the production of human immunodeficiency virus-1 (HIV-1)-derived lentiviral vectors and for the generation of transgenic rats by injection of these vectors into the perivitelline space of fertilized one-cell eggs.

In Vivo Gene Transfer by Electroporation Allows Expression of a Fluorescent Transgene in Hamster Testis and Epididymal Sperm and Has No Adverse Effects upon Testicular Integrity or Sperm Quality1

The study of gene function in testis and sperm has been greatly assisted by transgenic mouse models. Recently, an alternative way of expressing transgenes in mouse testis has been developed that uses electroporation to introduce transgenes into the male germ cells. This approach has been successfully used to transiently express reporter genes driven by constitutive and testis-specific promoters. It has been proposed as an alternative method for studying gene function in testis and sperm, and as a novel way to create transgenic animals. However, the low levels and transient nature of transgene expression that can be achieved using this technique have raised concerns about its practical usefulness. It has also not been demonstrated in mammals other than mice. In this study, we show for the first time that in vivo gene transfer using electroporation can be used to express a fluorescent transgene in the testis of a mammal other than mice, the Syrian golden hamster. Significantly, for the first time we demonstrate expression of a transgene in epididymal sperm using this approach. We show that expression of the transgene can be detected in sperm for as long as 60 days following gene transfer. Finally, we provide the first systematic demonstration that this technique does not lead to any significant long-term adverse effects on testicular integrity and sperm quality. This technique therefore offers a novel way to study gene function during fertilization in hamsters and may also have potential as a way of creating transgenic versions of this important model species.

In vivo gene transfer studies on the regulation and function of the vasopressin and oxytocin genes

Novel genes can be introduced into the germline of rats and mice by microinjecting fertilized one-cell eggs with fragments of cloned DNA. A gene sequence can thus be studied within the physiological integrity of the resulting transgenic animals, without any prior knowledge of its regulation and function. These technologies have been used to elucidate the mechanisms by which the expression of the two genes in the locus that codes for the neuropeptides vasopressin and oxytocin is confined to, and regulated physiologically within, specific groups of neurones in the hypothalamus. A number of groups have described transgenes, derived from racine, murine and bovine sources, in both rat and mouse hosts, that mimic the appropriate expression of the endogenous vasopressin and genes in magnocellular neurones (MCNs) of the supraoptic and paraventricular nuclei. However, despite considerable effort, a full description of the cis-acting sequences mediating the regulation of the vasopressin-oxytocin locus remains elusive. Two general conclusions have nonetheless been reached. First, that the proximal promoters of both genes are unable to confer any cell-specific regulatory controls. Second, that sequences downstream of the promoter, within the structural gene and/or the intergenic region that separates the two genes, are crucial for appropriate expression. Despite these limitations, sufficient knowledge has been garnered to specifically direct the expression of reporter genes to vasopressin and oxytocin MCNs. Further, it has been shown that reporter proteins can be directed to the regulated secretory pathway, from where they are subject to appropriate physiological release. The use of MCN expression vectors will thus enable the study of the physiology of these neurones through the targeted expression of biologically active molecules. However, the germline transgenic approach has a number of limitations involving the interpretation of phenotypes, as well as the large cost, labour and time demands. High-throughput somatic gene transfer techniques, principally involving the stereotaxic injection of hypothalamic neuronal groups with replication-deficient adenoviral vectors, are now being developed that obviate these difficulties, and which enable the robust, long-lasting expression of biologically active proteins in vasopressin and oxytocin MCNs.

Further delineation of the sequences required for the expression and physiological regulation of the vasopressin gene in transgenic rat hypothalamic magnocellular neurones

We have introduced transgenes into rats with a view to defining genomic regions that mediate the cell-specific and physiological regulation of the vasopressin gene. These transgenes consist of the rat vasopressin structural gene with a reporter inserted into exon III, flanked by different lengths of upstream and downstream sequences. 11-VCAT-3 is flanked by 11 kbp of upstream sequences and 3 kbp of downstream sequences. The previously described 5-VCAT-3 is flanked by 5 kbp of upstream and 3 kbp of downstream sequences. 3-VCAT-3 has 3 kbp of upstream and 3 kbp of downstream sequences, and 3-VCAT-0.2 is flanked by 3 kbp of upstream and 0.2 kbp of downstream sequences. All four transgenes elicit the same expression patterns; low basal expression is seen in the magnocellular supraoptic and paraventricular nuclei, and is negligible in the suprachiasmatic nucleus. Expression increases markedly in vasopressin magnocellular cells following dehydration. The sequences responsible for the cell-specific expression and physiological regulation of our transgenes thus reside within the confines of the smallest construct studied, 3-VCAT-0.2.

Comparison between PMSG- and FSH-induced superovulation for the generation of transgenic rats

Superovulation protocols using single injections of pregnant mare's serum gonadotropin (PMSG) or minipumps with follicle-stimulating hormone (FSH) were compared in immature Sprague-Dawley (SD) rats. We used the following criteria: total number of ova, rate of fertilization, in vitro embryo development, sensitivity of zygotes to the microinjection of foreign DNA into the pronucleus, and their in-vivo development after transplantation into the oviduct of a recipient. Female SD rats were stimulated with 15 IU PMSG or 10 mg FSH followed by the injection of human chorionic gonadotropin (hCG) at doses of 20 and 30 IU per female. After hCG administration, they were mated with males of the same strain and sacrificed on day 1 of pregnancy. The percentage of mated animals and the fertilization rate was similar in all groups. In rats given PMSG, the number of ovulated zygotes was hCG dose-dependent. In contrast, the dose of hCG did not influence the efficiency of superovulation in rats given FSH, which was equal to PMSG-treated rats at the optimal dose of hCG. The rates of in vitro blastocyst development (31.4 and 23.3%) and the resistance to microinjection into the pronucleus did also not differ significantly between zygotes of both studied groups. The proportion of offspring developing from microinjected zygotes after oviduct transfer (26.2 and 26.8%, respectively) and the rate of transgene integration per newborns (7.3 and 4.9%, respectively) was similar in both experimental groups. The results of this study demonstrate that superovulation of immature SD rats by PMSG is equally effective as FSH treatment and, thus, preferable for transgenic rat technology due to the lower costs and easier handling.