Direct comparison between ICSI-mediated DNA transfer and pronuclear DNA microinjection for producing transgenic rats

Germline transgenesis in rodents by pronuclear microinjection of Sleeping Beauty transposons

We describe a protocol for high-efficiency germline transgenesis and sustained transgene expression in two important biomedical models, the mouse and the rat, by using the Sleeping Beauty transposon system. The procedure is based on co-injection of synthetic mRNA encoding the SB100X hyperactive transposase, together with circular plasmid DNA carrying a transgene construct flanked by binding sites for the transposase, into the pronuclei of fertilized oocytes. Upon translation of the transposase mRNA, enzyme-mediated excision of the transgene cassettes from the injected plasmids followed by permanent genomic insertion produces stable transgenic animals. Generation of a germline-transgenic founder animal by using this protocol takes ∼3 months. Transposon-mediated transgenesis compares favorably in terms of both efficiency and reliable transgene expression with classic pronuclear microinjection, and it offers comparable efficacies to lentiviral approaches without limitations on vector design, issues of transgene silencing, and the toxicity and biosafety concerns of working with viral vectors.

Generation of transgenic rats by ooplasmic injection of sperm cells exposed to exogenous DNA

Intracytoplasmic sperm injection (ICSI) has been successfully achieved in mice and rats using a piezo-driven injection pipette, with the offspring rate of >30%. The ICSI technique was applied not only to rescue infertile male strains but also to produce transgenic rodents. The ICSI-mediated DNA transfer, that the sperm heads and exogenous DNA solution are mixed and co-injected into ooplasm, has been equally effective to the conventional pronuclear DNA microinjection. Production efficiency of transgenic founders by the ICSI-mediated DNA transfer was comparable between mice and rats, while the optimal DNA concentration was lower in rats than mice. The production efficiency was improved when membrane structure of sperm heads was partially disrupted by detergent or ultrasonic treatment before exposure to the exogenous DNA solution. Exogenous DNAs with various chain lengths were stably integrated into the rodent genomes of various genetic backgrounds by this method. The ICSI-mediated DNA transfer in which the preparation of pronuclear-stage fertilized zygotes is not required would be alternative to conventional pronuclear DNA microinjection.

Beyond the rat models of human neurodegenerative disorders

The rat is a model of choice in biomedical research for over a century. Currently, the rat presents the best "functionally" characterized mammalian model system. Despite this fact, the transgenic rats have lagged behind the transgenic mice as an experimental model of human neurodegenerative disorders. The number of transgenic rat models recapitulating key pathological hallmarks of Alzheimer's disease, Huntington's disease, amyotrophic lateral sclerosis, or human tauopathies is still limited. The reason is that the transgenic rats remain more difficult to produce than transgenic mice. The gene targeting technology is not yet established in rats due to the lack of truly totipotent embryonic stem cells and cloning technology. This extremely powerful technique has given the mouse a clear advantage over the rat in generation of new transgenic models. Despite these limitations, transgenic rats have greatly expanded the range of potential experimental approaches. The large size of rats permits intrathecal administration of drugs, stem cell transplantation, serial sampling of the cerebrospinal fluid, microsurgical techniques, in vivo nerve recordings, and neuroimaging procedures. Moreover, the rat is routinely employed to demonstrate therapeutic efficacy and to assess toxicity of novel therapeutic compounds in drug development. Here we suggest that the rat constitutes a slightly underestimated but perspective animal model well-suited for understanding the mechanisms and pathways underlying the human neurodegenerative disorders.

Gene therapy and intracytoplasmatic sperm injection (ICSI) - a review

Human gene therapy (HGT), the treatment or prevention of disease by gene transfer is, regarded by many, as a potential revolution in medicine, because gene therapies target the causes of disease, whereas most current drugs treat the symptoms. Micro-assisted fertilization in the form of intracytoplasmatic sperm injection (ICSI) has truly revolutionized the treatment options for couples with impaired semen quality, and those with both obstructive and non-obstructive azoospermia. ICSI involves the injection of a single sperm directly into the cytoplasm of a mature egg (oocyte) using a glass needle (pipette). Fertilization with this technique occurs in 50%-80% of injected oocytes, but may damage a small percentage of them. With gene therapy, there are new and varied strategies for gene transfer and genome sequence manipulation with improved methodologies that use the technique of microinjection such as the intracytoplasmatic sperm injection-mediated transgenesis (ICSI-Tr), active transgenesis or the pronuclear microinjection technique. This review will look at these methods as well as their potential applications and limitations.