Yujiro Kameyama

An accumulative site‐specific gene integration system using cre recombinase‐mediated cassette exchange

The Cre‐loxP system is frequently used for site‐specific recombination in animal cells. The equilibrium and specificity of the recombination reaction can be controlled using mutated loxPs. In the present study, we designed an accumulative site‐specific gene integration system using Cre recombinase and mutated loxPs in which the Cre‐mediated cassette exchange reaction is infinitely repeatable for target gene integration into loxP target sites. To evaluate the feasibility and usefulness of this system, a series of integration reactions were repeated and confirmed in vitro using Cre recombinase protein and plasmids. Accumulative gene integration was also performed on the genome of Chinese hamster ovary (CHO) cells. The results indicated that the system was applicable for repeated gene integration of multiple genes to the target sites on both plasmids and CHO cell genomes. This gene integration system provides a novel strategy for gene amplification and for biological analyses of gene function through the genetic modification of cells and organisms. Biotechnol. Bioeng. 2010;105: 1106–1114.

Repeated integration of antibody genes into a pre-selected chromosomal locus of CHO cells using an accumulative site-specific gene integration system

We previously reported an accumulative site-specific gene integration system using Cre recombinase and mutated loxP sites, where a recombinase-mediated cassette exchange (RMCE) reaction is repeatable. This gene integration system was applied for antibody production using recombinant Chinese hamster ovary (CHO) cells. We introduced an exchange cassette flanked by wild-type and mutated loxP sites into the chromosome of CHO cells for the establishment of recipient founder cells. Then, the donor plasmids including an expression cassette for an antibody gene flanked by a compatible pair of loxP sites were prepared. The donor plasmid and a Cre expression vector were co-transfected into the founder CHO cells to give rise to RMCE in the CHO genome, resulting in site-specific integration of the antibody gene. The RMCE procedure was repeated to increase the copy numbers of the integrated gene. Southern blot and genomic PCR analyses for the established cells revealed that the transgenes were integrated into the target site. Antibody production determined by ELISA and western blotting was increased corresponding to the number of transgenes. These results indicate that the accumulative site-specific gene integration system could provide a useful tool for increasing the productivity of recombinant proteins.

Accumulative gene integration into a pre-determined site using Cre/loxP

Site-specific gene recombination systems, such as Cre/loxP, have been used for genetic modification of cells and organisms in both basic and applied research. We previously developed an accumulative gene integration system (AGIS), in which target gene cassettes could be repeatedly integrated into a pre-determined site on a plasmid or cellular genome by recombinase-mediated cassette exchange (RMCE), using Cre and mutated loxPs. In the present study, we designed a simplified AGIS. For gene integration into a target site, the previous system used two loxP sites in the acceptor DNA, whereas the new system uses a single loxP site. The gene integration reactions were repeated four times in vitro using Cre protein and specific plasmids. The expected integration reactions mediated by Cre occurred at the loxP sites, resulting in integration of four target genes. The system was also used for genomic integration of reporter genes using Chinese hamster ovary (CHO) cells. The reporter genes were efficiently introduced into the CHO genome in a Cre-dependent manner, and transgene expression was detected after the integration reaction. The expression levels of the reporter genes were enhanced, corresponding to the increase of transgene copy number. Recombinase-mediated AGIS provides a useful tool for the modification of cellular genomes.

Magnetic Manipulation of a Retroviral Vector Using Magnetite Cationic Liposomes

For tissue engineering purposes, retroviral vectors represent an efficient method of delivering exogenous genes such as growth factors to injured tissues because gene-transduced cells can produce stable and constant levels of the gene product. However, retroviral vector technology suffers from low yields. In the present study, we used magnetite nanoparticles and magnetic force to concentrate the retroviral vectors to enhance the transduction efficiency and to enable their magnetic manipulation. Magnetite nanoparticles modified with cationic liposomes were added to a solution containing a retroviral vector pseudotyped with vesicular stomatitis virus glycoprotein. The magnetic particles that captured the viral vectors were collected using a magnetic force and seeded into mouse neuroblastoma Neuro2a cells. The viral titer was up to 55 times greater (up to 3 x 10(8) infectious units/mL). Additionally, the magnetically labeled retroviral vectors can be directed to the desired regions for infection by applying magnetic fields, and micro-patterns of gene-transduced cell regions could be created on a cellular monolayer using micro-patterned magnetic concentrators. These results suggest that this technique provides a promising approach to capturing and concentrating viral vectors, thus achieving high transduction efficiency and the ability to deliver genes to a specific injured site by applying a magnetic field.

Improved transgene integration into the Chinese hamster ovary cell genome using the Cre-loxP system.

Genetic engineering of cellular genomes has provided useful tools for biomedical and pharmaceutical studies such as the generation of transgenic animals and producer cells of biopharmaceutical proteins. Gene integration using site-specific recombinases enables precise transgene insertion into predetermined genomic sites if the target site sequence is introduced into a specific chromosomal locus. We previously developed an accumulative site-specific gene integration system (AGIS) using Cre and mutated loxPs. The system enabled the repeated integration of multiple transgenes into a predetermined locus of a genome. In this study, we explored applicable mutated loxP pairs for AGIS to improve the integration efficiency. The integration efficiencies of 52 mutated loxP sequences, including novel sequences, were measured using an in vitro evaluation system. Among mutated loxP pairs that exhibited a high integration efficiency, the applicability of the selected pairs to AGIS was confirmed for transgene integration into the Chinese hamster ovary cell genome. The newly found mutated loxP pairs should be useful for Cre-mediated integration of transgenes and AGIS.

Antibody-dependent gene transduction using gammaretroviral and lentiviral vectors pseudotyped with chimeric vesicular stomatitis virus glycoprotein

Gammaretroviral and lentiviral vectors pseudotyped with vesicular stomatitis virus glycoprotein G (VSV-G) have been used for stable gene transfer because of their broad host range and high mechanical strength. In the present study, an expression plasmid for chimeric VSV-G, consisting of a ZZ fragment derived from Staphylococcal protein A fused to the N-terminus of VSV-G (ZZ-VSV-G), was constructed to produce viral vectors capable of antibody-dependent gene transduction. Gammaretroviral (based on mouse stem cell virus, MSCV) and lentiviral (based on human immunodeficiency virus type 1, HIV-1) vectors pseudotyped with ZZ-VSV-G were produced without the loss of antibody-binding activity. The production of infectious viral particles was promoted by the addition of an expression plasmid for native VSV-G and antibody-dependent gene transduction was achieved using plates coated with antibodies. This system may be useful for the genetic transduction of cells expressing specific proteins on their surface, and for screening of antibodies specific for cell surface receptors.

Retroviral Vectors Pseudotyped with Chimeric Vesicular Stomatitis Virus Glycoprotein for Antibody-Dependent Gene Transduction

Retroviral vectors are a powerful tool for stable gene transfer in animal cells because of their ability to integrate the target gene into the host genome. Above all, retroviral vectors pseudotyped with vesicular stomatitis virus glycoprotein G (VSV-G) have been used widely because of their broad host range specificity. These vectors can infect various cells regardless of species (pantropic), since the receptors for VSV-G are phospholipids which are observed ubiquitously in membrane lipids. In addition, pseudotyping with VSV-G improves the mechanical strength of viral vectors, and viral titers can be increased by ultracentrifugation. However, the in vivo use of pantropic viral vectors is limited due to the low transduction efficiency of target cells and possible side-effects. In the present study, an expression plasmid for chimeric VSV-G, consisting of a ZZ fragment derived from Staphylococcal protein A fused to the N-terminus of VSV-G (ZZ-VSV-G), was constructed to produce viral vectors capable of antibody-dependent gene transduction. Gammaretroviral and lentiviral vectors pseudotyped with ZZ-VSV-G were produced without the loss of antibody-binding activity. The production of infectious viral particles was promoted by the addition of an expression plasmid for native VSV-G and antibody-dependent gene transduction was achieved using plates coated with antibodies. This system may be useful for genetic transduction of cells expressing specific proteins on their surface, and for screening of antibodies specific for cell surface receptors.