Nuclear targeting determinants of the phage P1 cre DNA recombinase

Herpes simplex virus type 1 impairs mucosal-associated invariant T cells

Herpes simplex virus type 1 (HSV-1) is a highly successful pathogen that infects mucosal sites and adopts an arsenal of strategies to manipulate host immunity. Mucosal-associated invariant T (MAIT) cells are abundant innate-like T lymphocytes that recognize bacterial and fungal-derived vitamin B-related metabolites presented by major histocompatibility complex class I-related protein 1 (MR1). MAIT cells can also be activated in an MR1-independent manner via cytokine stimulation, predominantly by IL-12 and IL-18. MAIT cell alterations have been identified as being associated with a number of viral infections, but direct interactions between viruses and MAIT cells are poorly understood. It is unknown whether HSV-1 can infect MAIT cells and modulate their functions. Here, we show that HSV-1 can infect primary human MAIT cells, including CD4±/CD8± and CD56± MAIT cell subpopulations. Furthermore, HSV-1 infection profoundly inhibits the functional capacity of MAIT cells to respond to T cell receptor (TCR)-dependent stimulation by the MAIT cell activating ligand 5-(2-oxopropylideneamino)-6-D-ribitylaminouracil (5-OP-RU) and to cytokine stimulation by IL-12/IL-18. HSV-1-infected MAIT cells display reduced cytotoxic potential, diminished synthesis of effector cytokines, and decreased expression of key cytokine receptors including IL-18R. In addition, MAIT cells exposed to HSV-1-infected fibroblasts but which remained uninfected (viral GFP-negative) also exhibit a suppressed effector response to TCR-dependent stimulation. The functional suppression of HSV-1-exposed MAIT cells was not mediated by a soluble factor within the supernatant, suggesting direct contact of MAIT cells with HSV-1-infected fibroblasts is required. Overall, this study reveals that HSV-1 can infect MAIT cells and substantially impair MAIT cell effector functions.

IMPORTANCE

Mucosal-associated invariant T cells (MAIT cells) are "unconventional" immune cells that are becoming increasingly appreciated to play important roles in a variety of viral infections. Herpes simplex virus (HSV) causes significant human disease and is a master manipulator of multiple immune functions, but how this virus may control MAIT cells is poorly understood. We discovered that HSV can infect human MAIT cells and impair their functional capacity and also show that MAIT cells exposed to HSV, but which do not show evidence of infection, are similarly impaired. This study therefore identifies an additional immunomodulatory function of HSV.

Dynamics in Cre-loxP site-specific recombination

Cre recombinase is a phage-derived enzyme that has found utility for precise manipulation of DNA sequences. Cre recognizes and recombines pairs of loxP sequences characterized by an inverted repeat and asymmetric spacer. Cre cleaves and religates its DNA targets such that error-prone repair pathways are not required to generate intact DNA products. Major obstacles to broader applications are lack of knowledge of how Cre recognizes its targets, and how its activity is controlled. The picture emerging from high resolution methods is that the dynamic properties of both the enzyme and its DNA target are important determinants of its activity in both sequence recognition and DNA cleavage. Improved understanding of the role of dynamics in the key steps along the pathway of Cre-loxP recombination should significantly advance our ability to both redirect Cre to new sequences and to control its DNA cleavage activity in the test tube and in cells.

Platform establishment of the Cre-loxP recombination system for genetic manipulation of the Lumpy skin disease virus

Lumpy skin disease virus (LSDV) is a rapidly emerging pathogen in Asia, including China. Genetic manipulation of the LSDV is essential for the elucidation of the pathogenic mechanism and biological function of the LSDV-encoded protein. In this study, we established a platform for the Cre-loxP recombination system under a modified early-late H5 promoter of the VACV for quick construction of the recombinant LSDV virus. The recombinant virus, LSDV-EGFP-ΔTK, was purified and obtained using serial limited dilution and picking the single cells methods. Using the lentiviral package system, a Cre recombinase enzyme stable expression MDBK cell line was established to supply the Cre recombinase for the reporter gene excision. A genetically stable, safe TK gene-deleted LSDV (LSDV-ΔTK) was constructed using homologous recombination and the Cre-loxP system. It was purified using limited dilution in the MDBK-Cre cell line. Establishing the Cre-loxP recombination system will enable sequential deletion of the interested genes from the LSDV genome and genetic manipulation of the LSDV genome, providing technical support and a platform for developing the attenuated LSDV vaccine.

Application of a split-Cre system for high-capacity adenoviral vector amplification

BACKGROUND AND AIMS

High-capacity adenoviral vectors (HC-AdV) show extended DNA payload and stability of gene expression in vivo due to the absence of viral coding sequences. However, production requires methods to trans-complement viral proteins, usually through Helper Viruses (HV). The Cre/loxP system is frequently employed to remove the packaging signal in HV genomes, in order to avoid their encapsidation. However, chronic exposure to the Cre recombinase in packaging cells is detrimental. We have applied the dimerizable Cre system to overcome this limitation.

METHODS AND RESULTS

Cre was split in two fragments devoid of recombinase function (N-terminal 244 and C-terminal 99 amino-acids). In one version of the system, interaction with both moieties was favored by rapamycin-dependent heterodimerization domains (DiCre). Other version contained only Cre sequences (oCre). We generated packaging cells and HVs expressing the complementary fragments and studied their performance for HC-AdV production. We found that both conformations avoided interference with the growth of packaging cells, and the oCre system was particularly suitable for HC-AdV amplification.

CONCLUSIONS

The split-Cre system improves the performance of packaging cells and can reduce the time and cost of HC-AdV amplification up to 30% and 15%, respectively. This may contribute to the standardization of HC-AdV production.

Multiplication of defective Ebola virus in a complementary permissive cell line

In an effort to develop safe and innovative in vitro models for Ebola virus (EBOV) research, we generated a recombinant Ebola virus where the glycoprotein (GP) gene was substituted with the Cre recombinase (Cre) gene by reverse genetics. This defective virus could multiply itself in a complementary permissive cell line, which could express GP and reporter protein upon exogenous Cre existence. The main features of this novel model for Ebola virus are intact viral life cycle, robust virus multiplication and normal virions morphology. The design of this model ensures its safety, excellent stability and maneuverability as a tool for virology research as well as for antiviral agent screening and drug discovery, and such a design could be further adapted to other viruses.

Imaging of anthrax intoxication in mice reveals shared and individual functions of surface receptors CMG-2 and TEM-8 in cellular toxin entry

Bacillus anthracis lethal toxin and edema toxin are binary toxins that consist of a common cell-binding moiety, protective antigen (PA), and the enzymatic moieties, lethal factor (LF) and edema factor (EF). PA binds to either of two receptors, capillary morphogenesis protein-2 (CMG-2) or tumor endothelial marker-8 (TEM-8), which triggers the binding and cytoplasmic translocation of LF and EF. However, the distribution of functional TEM-8 and CMG-2 receptors during anthrax toxin intoxication in animals has not been fully elucidated. Herein, we describe an assay to image anthrax toxin intoxication in animals, and we use it to visualize TEM-8- and CMG-2-dependent intoxication in mice. Specifically, we generated a chimeric protein consisting of the N-terminal domain of LF fused to a nuclear localization signal-tagged Cre recombinase (LFn-NLS-Cre). When PA and LFn-NLS-Cre were coadministered to transgenic mice expressing a red fluorescent protein in the absence of Cre and a green fluorescent protein in the presence of Cre, intoxication could be visualized at single-cell resolution by confocal microscopy or flow cytometry. Using this assay, we found that: (a) CMG-2 is critical for intoxication in the liver and heart, (b) TEM-8 is required for intoxication in the kidney and spleen, (c) CMG-2 and TEM-8 are redundant for intoxication of some organs, (d) combined loss of CMG-2 and TEM-8 completely abolishes intoxication, and (e) CMG-2 is the dominant receptor on leukocytes. The novel assay will be useful for basic and clinical/translational studies of Bacillus anthracis infection and for clinical development of reengineered toxin variants for cancer treatment.

Astrocyte-to-neuron transportation of enhanced green fluorescent protein in cerebral cortex requires F-actin dependent tunneling nanotubes

Tunneling nanotube (TNT), a dynamic cell–cell contact, is dependent on actin polymerization. TNTs are efficient in transporting ions, proteins and organelles intercellularly, which are important mechanisms in physiological and pathological processes. Reported studies on the existence and function of TNTs among neural cells focus on cultured cell for the convenience in detecting TNTs’ ultrastructure. In this study, the adeno-associated virus (AAV-GFAP-EGFP-p2A-cre) was injected into the cerebral cortex of knock-in mice ROSA26 GNZ. GFAP promoter initiated the expression of enhanced green fluorescent protein (EGFP) in infected astrocytes. At 10 days post injection (10 DPI), EGFP transferred from astrocytes in layer I–III to neurons in layer V. The dissemination of EGFP was not through endocytosis or exosome. Applying microscopes, we found that the intercellular transportation of EGFP through contact connection was F-actin dependent. Therefore, we concluded that EGFP transported from astrocytes to neurons in cortex via F-actin dependent TNTs. This study first proved that proteins transported intercellularly via TNTs in brain.

Precise optical control of gene expression in C. elegans using improved genetic code expansion and Cre recombinase

Synthetic strategies for optically controlling gene expression may enable the precise spatiotemporal control of genes in any combination of cells that cannot be targeted with specific promoters. We develop an improved genetic code expansion system in Caenorhabditis elegans and use it to create a photoactivatable Cre recombinase. We laser-activate Cre in single neurons within a bilaterally symmetric pair to selectively switch on expression of a loxP-controlled optogenetic channel in the targeted neuron. We use the system to dissect, in freely moving animals, the individual contributions of the mechanosensory neurons PLML/PLMR to the C. elegans touch response circuit, revealing distinct and synergistic roles for these neurons. We thus demonstrate how genetic code expansion and optical targeting can be combined to break the symmetry of neuron pairs and dissect behavioural outputs of individual neurons that cannot be genetically targeted.

Animal behaviour and movement emerges from the stimulation of nerve cells that are connected together like a circuit. Researchers use various tools to investigate these neural networks in model organisms such as roundworms, fruit flies and zebrafish. The trick is to activate some nerve cells, but not others, so as to isolate their specific role within the neural circuit.

One way to do this is to switch genes on or off in individual cells as a way to control their neuronal activity. This can be achieved by building a photocaged version of the enzyme Cre recombinase which is designed to target specific genes. The modified Cre recombinase contains an amino acid (the building blocks of proteins) that inactivates the enzyme. When the cell is illuminated with UV light, a part of the amino acid gets removed allowing Cre recombinase to turn on its target gene.

However, cells do not naturally produce these photocaged amino acids. To overcome this, researchers can use a technology called genetic code expansion which provides cells with the tools they need to build proteins containing these synthetic amino acids. Although this technique has been used in live animals, its application has been limited due to the small amount of proteins it produces. Davis et al. therefore set out to improve the efficiency of genetic code expansion so that it can be used to study single nerve cells in freely moving roundworms.

In the new system, named LaserTAC, individual cells are targeted with UV light that ‘uncages’ the Cre recombinase enzyme so it can switch on a gene for a protein that controls neuronal activity. Davis et al. used this approach to stimulate a pair of neurons sensitive to touch to see how this impacted the roundworm’s behaviour. This revealed that individual neurons within this pair contribute to the touch response in different ways. However, input from both neurons is required to produce a robust reaction.

These findings show that the LaserTAC system can be used to manipulate gene activity in single cells, such as neurons, using light. It allows researchers to precisely control in which cells and when a given gene is switched on or off. Also, with the improved efficiency of the genetic code expansion, this technology could be used to modify proteins other than Cre recombinase and be applied to other artificial amino acids that have been developed in recent years.

Phage Therapy: Beyond Antibacterial Action

Until recently, phages were considered as mere “bacteria eaters” with potential for use in combating antimicrobial resistance. The real value of phage therapy assessed according to the standards of evidence-based medicine awaits confirmation by clinical trials. However, the progress in research on phage biology has shed more light on the significance of phages. Accumulating data indicate that phages may also interact with eukaryotic cells. How such interactions could be translated into advances in medicine (especially novel means of therapy) is discussed herein.

Efficient delivery of genome-editing proteins using bioreducible lipid nanoparticles

A central challenge to the development of protein-based therapeutics is the inefficiency of delivery of protein cargo across the mammalian cell membrane, including escape from endosomes. Here we report that combining bioreducible lipid nanoparticles with negatively supercharged Cre recombinase or anionic Cas9:single-guide (sg)RNA complexes drives the electrostatic assembly of nanoparticles that mediate potent protein delivery and genome editing. These bioreducible lipids efficiently deliver protein cargo into cells, facilitate the escape of protein from endosomes in response to the reductive intracellular environment, and direct protein to its intracellular target sites. The delivery of supercharged Cre protein and Cas9:sgRNA complexed with bioreducible lipids into cultured human cells enables gene recombination and genome editing with efficiencies greater than 70%. In addition, we demonstrate that these lipids are effective for functional protein delivery into mouse brain for gene recombination in vivo. Therefore, the integration of this bioreducible lipid platform with protein engineering has the potential to advance the therapeutic relevance of protein-based genome editing.

Preparation of cell-permeable Cre recombinase by expressed protein ligation

Background

Protein transduction is safer than viral vector-mediated transduction for the delivery of a therapeutic protein into a cell. Fusion proteins with an arginine-rich cell-penetrating peptide have been produced in E. coli, but the low solubility of the fusion protein expressed in E. coli impedes the large-scale production of fusion proteins from E. coli.

Results

Expressed protein ligation is a semisynthetic method to ligate a bacterially expressed protein with a chemically synthesized peptide. In this study, we developed expressed protein ligation-based techniques to conjugate synthetic polyarginine peptides to Cre recombinase. The conjugation efficiency of this technique was higher than 80%. Using this method, we prepared semisynthetic Cre with poly-L-arginine (ssCre-R9), poly-D-arginine (ssCre-dR9) and biotin (ssCre-dR9-biotin). We found that ssCre-R9 was delivered to the cell to a comparable level or more efficiently compared with Cre-R11 and TAT-Cre expressed as recombinant fusion proteins in E. coli. We also found that the poly-D-arginine cell-penetrating peptide was more effective than the poly-L-arginine cell-penetrating peptide for the delivery of Cre into cell. We visualized the cell transduced with ssCre-dR9-biotin using avidin-FITC.

Conclusions

Collectively, the results demonstrate that expressed protein ligation is an excellent technique for the production of cell-permeable Cre recombinase with polyarginine cell-penetrating peptides. In addition, this approach will extend the use of cell-permeable proteins to more sophisticated applications, such as cell imaging.

Electronic supplementary material

The online version of this article (doi:10.1186/s12896-015-0126-z) contains supplementary material, which is available to authorized users.

Mitochondrial oxidative stress in the retinal pigment epithelium leads to localized retinal degeneration

PURPOSE

Oxidative stress in the RPE is widely accepted as a contributing factor to AMD. We have previously shown that ribozyme-mediated reduction in the antioxidant enzyme manganese superoxide dismutase (MnSOD) leads to some of the features of geographic atrophy in mice. To develop a mouse model independent of viral injection, we used a conditional knockout of the Sod2 gene in the RPE to elevate mitochondrial oxidative stress in that cell layer.

METHODS

Experimental mice in which exon 3 of Sod2 was flanked by loxP sites were also transgenic for PVMD2-rtTA and tetO-PhCMV cre, so that cre recombinase was expressed only in the RPE. Pups of this genotype (Sod2(flox/flox)VMD2cre) were induced to express cre recombinase by feeding doxycycline-laced chow to nursing dams. Controls included mice of this genotype not treated with doxycycline and doxycycline-treated Sod2(flox/flox) mice lacking the cre transgene. Expression of cre in the RPE was verified by immunohistochemistry, and deletion of Sod2 exon 3 in the RPE was confirmed by PCR. Mice were followed up over a period of 9 months by spectral-domain optical coherence tomography (SD-OCT), digital fundus imaging, and full-field ERG. Following euthanasia, retinas were examined by light and electron microscopy or by immunohistochemistry. Contour length of rod outer segments and thickness of the RPE layer were measured by unbiased stereology.

RESULTS

Following doxycycline induction of cre, Sod2(flox/flox) cre mice demonstrated increased signs of oxidative stress in the RPE and accumulation of autofluorescent material by age 2 months. They showed a gradual decline in the ERG response and thinning of the outer nuclear layer (by SD-OCT), which were statistically significant by 6 months. In addition, OCT and electron microscopy revealed increased porosity of the choroid. At the same interval, hypopigmented foci appeared in fundus micrographs, and vascular abnormalities were detected by fluorescein angiography. By 9 months, the RPE layer in Sod2(flox/flox) cre mice was thicker than in nontransgenic littermates, and the rod outer segments were significantly longer over most of the retina, although localized atrophy of photoreceptors was also obvious in some eyes.

CONCLUSIONS

Conditional tissue-specific reduction in MnSOD induced oxidative stress in mouse RPE, leading to RPE dysfunction, damage to the choroid, and death of photoreceptor cells. The RPE oxidative stress did not cause drusen-like deposits, but the model recapitulated certain key aspects of the pathology of dry AMD and may be useful in testing therapies.

Bacteriophages as vehicles for gene delivery into mammalian cells: prospects and problems

INTRODUCTION

The identification of more efficient gene delivery vehicles (GDVs) is essential to fulfill the expectations of clinical gene therapy. Bacteriophages, due to their excellent safety profile, extreme stability under a variety of harsh environmental conditions and the capability for being genetically manipulated, have drawn a flurry of interest to be applied as a newly arisen category of gene delivery platforms.

AREAS COVERED

The incessant evolutionary interaction of bacteriophages with human cells has turned them into a part of our body's natural ecosystem. However, these carriers represent several barriers to gene transduction of mammalian cells. The lack of evolvement of specialized machinery for targeted cellular internalization, endosomal, lysosomal and proteasomal escape, cytoplasmic entry, nuclear localization and intranuclear transcription poses major challenges to the expression of the phage-carried gene. In this review, we describe pros and cons of bacteriophages as GDVs, provide an insight into numerous barriers that bacteriophages face for entry into and subsequent trafficking inside mammalian cells and elaborate on the strategies used to bypass these barriers.

EXPERT OPINION

Tremendous genetic flexibility of bacteriophages to undergo numerous surface modifications through phage display technology has proven to be a turning point in the uncompromising efforts to surmount the limitations of phage-mediated gene expression. The revelatory outcomes of the studies undertaken within the recent years have been promising for phage-mediated gene delivery to move from concept to reality.

Nuclear and nucleoid localization are independently conserved functions in bacteriophage terminal proteins

Bacteriophage terminal proteins (TPs) prime DNA replication and become covalently linked to the DNA 5'-ends. In addition, they are DNA-binding proteins that direct early organization of phage DNA replication at the bacterial nucleoid and, unexpectedly, contain nuclear localization signals (NLSs), which localize them to the nucleus when expressed in mammalian cells. In spite of the lack of sequence homology among the phage TPs, these three properties share some common features, suggesting a possible evolutionary common origin of TPs. We show here that NLSs of three different phage TPs, Φ29, PRD1 and Cp-1, are mapped within the protein region required for nucleoid targeting in bacteria, in agreement with a previously proposed common origin of DNA-binding domains and NLSs. Furthermore, previously reported point mutants of Φ29 TP with no nuclear localization still can target the bacterial nucleoid, and Cp-1 TP contains two independent NLSs, only one of them required for nucleoid localization. Altogether, our results show that nucleoid and nucleus localization sequence requirements partially overlap, but they can be uncoupled, suggesting that conservation of both features could have a common origin but, at the same time, they have been independently conserved during evolution.

Less is more: strategies to remove marker genes from transgenic plants

Selectable marker genes (SMGs) and selection agents are useful tools in the production of transgenic plants by selecting transformed cells from a matrix consisting of mostly untransformed cells. Most SMGs express protein products that confer antibiotic- or herbicide resistance traits, and typically reside in the end product of genetically-modified (GM) plants. The presence of these genes in GM plants, and subsequently in food, feed and the environment, are of concern and subject to special government regulation in many countries. The presence of SMGs in GM plants might also, in some cases, result in a metabolic burden for the host plants. Their use also prevents the re-use of the same SMG when a second transformation scheme is needed to be performed on the transgenic host. In recent years, several strategies have been developed to remove SMGs from GM products while retaining the transgenes of interest. This review describes the existing strategies for SMG removal, including the implementation of site specific recombination systems, TALENs and ZFNs. This review discusses the advantages and disadvantages of existing SMG-removal strategies and explores possible future research directions for SMG removal including emerging technologies for increased precision for genome modification.

Genetic inhibition of Na+-Ca2+ exchanger current disables fight or flight sinoatrial node activity without affecting resting heart rate

RATIONALE

The sodium-calcium exchanger 1 (NCX1) is predominantly expressed in the heart and is implicated in controlling automaticity in isolated sinoatrial node (SAN) pacemaker cells, but the potential role of NCX1 in determining heart rate in vivo is unknown.

OBJECTIVE

To determine the role of Ncx1 in heart rate.

METHODS AND RESULTS

We used global myocardial and SAN-targeted conditional Ncx1 knockout (Ncx1(-/-)) mice to measure the effect of the NCX current on pacemaking activity in vivo, ex vivo, and in isolated SAN cells. We induced conditional Ncx1(-/-) using a Cre/loxP system. Unexpectedly, in vivo and ex vivo hearts and isolated SAN cells showed that basal rates in Ncx1(-/-) (retaining ≈20% of control level NCX current) and control mice were similar, suggesting that physiological NCX1 expression is not required for determining resting heart rate. However, increases in heart rate and SAN cell automaticity in response to isoproterenol or the dihydropyridine Ca(2+) channel agonist BayK8644 were significantly blunted or eliminated in Ncx1(-/-) mice, indicating that NCX1 is important for fight or flight heart rate responses. In contrast, the pacemaker current and L-type Ca(2+) currents were equivalent in control and Ncx1(-/-) SAN cells under resting and isoproterenol-stimulated conditions. Ivabradine, a pacemaker current antagonist with clinical efficacy, reduced basal SAN cell automaticity similarly in control and Ncx1(-/-) mice. However, ivabradine decreased automaticity in SAN cells isolated from Ncx1(-/-) mice more effectively than in control SAN cells after isoproterenol, suggesting that the importance of NCX current in fight or flight rate increases is enhanced after pacemaker current inhibition.

CONCLUSIONS

Physiological Ncx1 expression is required for increasing sinus rates in vivo, ex vivo, and in isolated SAN cells, but not for maintaining resting heart rate.

Functional eukaryotic nuclear localization signals are widespread in terminal proteins of bacteriophages

A number of prokaryotic proteins have been shown to contain nuclear localization signals (NLSs), although its biological role remains sometimes unclear. Terminal proteins (TPs) of bacteriophages prime DNA replication and become covalently linked to the genome ends. We predicted NLSs within the TPs of bacteriophages from diverse families and hosts and, indeed, the TPs of Φ29, Nf, PRD1, Bam35, and Cp-1, out of seven TPs tested, were found to localize to the nucleus when expressed in mammalian cells. Detailed analysis of Φ29 TP led us to identify a bona fide NLS within residues 1-37. Importantly, gene delivery into the eukaryotic nucleus is enhanced by the presence of Φ29 TP attached to the 5' DNA ends. These findings show a common feature of TPs from diverse bacteriophages targeting the eukaryotic nucleus and suggest a possible common function by facilitating the horizontal transfer of genes between prokaryotes and eukaryotes.

Use of the Cre-loxP recombination system as an estimate for Agrobacterium-mediated co-transformation of tobacco leaves

Agrobacterium tumefaciens-mediated transformation of tobacco leaves (Nicotiana tabacum) is used to study gene expression in a heterologous genetic background. Here, the Cre-loxP recombination system was used to detect T-DNA transfer by two A. tumefaciens cells harboring different binary vectors. Cre, under the control of the CaMV 35S promoter, was cloned into one vector, and a loxP cassette into another vector. A mixture of A. tumefaciens, in which each cell contained either a Cre- or loxP-vector, was co-infiltrated into tobacco leaves. After two days, excision of loxP-flanked DNA was detected by PCR and used as an estimate for co-transformation events. Strongest excision (> 50%) was observed when the loxP cassette was cloned into vector pPZP112 and Cre into pISV2678. This fast and easy technique can be used to assess the co-transformation efficiency of tobacco cells in future studies.

Nanobody-based chimeric receptor gene integration in Jurkat cells mediated by φC31 integrase

The crucial role of T lymphocytes in anti-tumor immunity has led to the development of novel strategies that can target and activate T cells against tumor cells. Recombinant DNA technology has been used to generate non-MHC-restricted chimeric antigen receptors (CARs). Here, we constructed a panel of recombinant CAR that harbors the anti-MUC1 nanobody and the signaling and co-signaling moieties (CD3ζ/CD28) with different spacer regions derived from human IgG3 with one or two repeats of the hinge sequence or the hinge region of FcγRII. The PhiC31 integrase system was employed to investigate if the recombination efficiency could be recruited for high and stable expression of T cell chimeric receptor genes. The effect of nuclear localization signal (NLS) and two different promoters (CMV and CAG) on efficacy of PhiC31 integrase in human T cell lines was evaluated. The presence of integrase in combination with NLS, mediated up to 7.6 and 8.5 fold increases in CAR expression in ZCHN-attB and ZCHHN-attB cassette integrated T cells, respectively. Our results showed that highly efficient and stable transduction of the Jurkat cell line by PhiC31 integrase is a feasible modality for generating anti-cancer chimeric T cells for use in cancer immunotherapy.

Differential, type I interferon-mediated autophagic trafficking of hepatitis C virus proteins in mouse liver

BACKGROUND & AIMS

The hepatitis C virus (HCV) serine protease NS3/4A can cleave mitochondria-associated antiviral signaling protein (MAVS) and block retinoic acid-inducible gene I-mediated interferon (IFN) responses. Although this mechanism is thought to have an important role in HCV-mediated innate immunosuppression, its significance in viral persistence is not clear.

METHODS

We generated transgenic mice that express the HCV NS3/4A proteins specifically in the liver and challenged the animals with a recombinant vesicular stomatitis virus, a synthetic HCV genome, IFN alfa, or IFN beta. We evaluated the effects of HCV serine protease on the innate immune responses and their interactions.

RESULTS

Expression of HCV NS3/4A resulted in cleavage of intrahepatic MAVS; challenge of transgenic mice with vesicular stomatitis virus or a synthetic HCV genome induced strong, type I IFN-mediated responses that were not significantly lower than those of control mice. Different challenge agents induced production of different ratios of IFN alfa and beta, resulting in different autophagic responses and vesicular trafficking patterns of endoplasmic reticulum- and mitochondria-associated viral proteins. IFN beta promoted degradation of the viral proteins by the autolysosome. Variant isoforms of MAVS were associated with distinct, type I IFN-mediated autophagic responses; these responses have a role in trafficking of viral components to endosomal compartments that contain Toll-like receptor-3.

CONCLUSIONS

IFN beta mediates a distinct autophagic mechanism of antiviral host defense. MAVS has an important role in type I IFN-induced autophagic trafficking of viral proteins.

Generating conditional knockout mice

Gene targeting in ES cells is extensively used to generate designed mouse mutants and to study gene function in vivo. Knockout mice that harbor a null allele in their germline provide appropriate genetic models of inherited diseases and often exhibit embryonic or early postnatal lethality. To study gene function in adult mice and in selected cell types, a refined strategy for conditional gene inactivation has been developed that relies on the DNA recombinase Cre and its recognition (loxP) sites. For conditional mutagenesis, a target gene is modified by the insertion of two loxP sites that enable to excise the flanked (floxed) gene segment through Cre-mediated recombination. Conditional mutant mice are obtained by crossing the floxed strain with a Cre transgenic line such that the target gene becomes inactivated in vivo within the expression domain of Cre. A large collection of Cre transgenic lines has been generated over time and can be used in a combinatorial manner to achieve gene inactivation in many different cell types. A growing number of CreER(T2) transgenic mice further allows for inducible inactivation of floxed alleles in adult mice upon administration of tamoxifen. This chapter covers the design and construction of loxP flanked alleles and refers to the vectors, ES cells, and mice generated by the European conditional mouse mutagenesis (EUCOMM) project. We further describe the design and use of Cre and CreER(T2) transgenic mice and a convenient breeding strategy to raise conditional mutants and controls for phenotype analysis.

Conditional gene targeting: dissecting the cellular mechanisms of retinal degenerations

Retinal neuron degeneration and survival are often regulated by the same trophic factors that are required for embryonic development and are usually expressed in multiple cell-types. Therefore, the conditional gene targeting approach is necessary to investigate the cell-specific function of widely expressed and developmentally regulated genes in retinal degeneration. The discussion in this review will be focused on the use of Cre/lox-based conditional gene targeting approach in mechanistic studies for retinal degeneration. In addition to the basic experimental designs, this article addresses various factors influencing the outcomes of conditional gene targeting studies, limitations of current technologies, availability of Cre-drive lines for various retinal cells, and issues related to the generation of Cre-expressing mice. Finally, this review will update the current status on the use of Cre/lox-based gene targeting approach in mechanistic studies for retinal degeneration, which includes rod photoreceptor survival under photo-oxidative stress and protein trafficking in photoreceptors.

Tus, an E. coli protein, contains mammalian nuclear targeting and exporting signals

Shuttling of proteins between nucleus and cytoplasm in mammalian cells is facilitated by the presence of nuclear localization signals (NLS) and nuclear export signals (NES), respectively. However, we have found that Tus, an E. coli replication fork arresting protein, contains separate sequences that function efficiently as NLS and NES in mammalian cell lines, as judged by cellular location of GFP-fusion proteins. The NLS was localized to a short stretch of 9 amino acids in the carboxy-terminus of Tus protein. Alterations of any of these basic amino acids almost completely abolished the nuclear targeting. The NES comprises a cluster of leucine/hydrophobic residues located within 21 amino acids at the amino terminus of Tus. Finally, we have shown that purified GFP-Tus fusion protein or GFP-Tus NLS fusion protein, when added to the culture media, was internalized very efficiently into mammalian cells. Thus, Tus is perhaps the first reported bacterial protein to possess both NLS and NES, and has the capability to transduce protein into mammalian cells.

Trafficking of membrane proteins to cone but not rod outer segments is dependent on heterotrimeric kinesin-II

Heterotrimeric kinesin-II is a molecular motor localized to the inner segment, connecting cilium and axoneme of mammalian photoreceptors. Our purpose was to identify the role of kinesin-II in anterograde intraflagellar transport by photoreceptor-specific deletions of kinesin family member 3A (KIF3A), its obligatory motor subunit. In cones lacking KIF3A, membrane proteins involved in phototransduction did not traffic to the outer segments resulting in complete absence of a photopic electroretinogram and progressive cone degeneration. Rod photoreceptors lacking KIF3A degenerated rapidly between 2 and 4 weeks postnatally, but the phototransduction components including rhodopsin trafficked to the outer segments during the course of degeneration. Furthermore, KIF3A deletion did not affect synaptic anterograde trafficking. The results indicate that trafficking of membrane proteins to the outer segment is dependent on kinesin-II in cone, but not rod photoreceptors, even though rods and cones share similar structures, and closely related phototransduction polypeptides.

Cre reconstitution allows for DNA recombination selectively in dual-marker-expressing cells in transgenic mice

Cre/LoxP-based DNA recombination has been used to introduce desired DNA rearrangements in various organisms, having for example, greatly assisted genetic analyses in mice. For most applications, single gene promoters are used to drive Cre production for conditional gene activation/inactivation or lineage-tracing experiments. Such a manipulation introduces Cre in all cells in which the utilized promoter is active. To overcome the limited selectivity of single promoters for cell-type-specific recombination, we have explored the ‘dual promoter combinatorial control’ of Cre activity, so that Cre activity could be restricted to cells that express dual protein markers. We efficiently reconstituted Cre activity from two modified, inactive Cre fragments. Cre re-association was greatly enhanced by fusing the Cre fragments separately to peptides that can form a tight antiparallel leucine zipper. The co-expressed Cre fusion fragments showed substantial activity in cultured cells. As proof of principle of the utility of this technique in vivo for manipulating genes specifically in dual-marker-positive cells, we expressed each inactive Cre fragments in transgenic mice via individual promoters. Result showed the effective reconstitution of Cre activates LoxP recombination in the co-expressing cells.

Novel recombination system using Cre recombinase alpha complementation

A major limitation for the use of Cre recombinase is its toxicity and a lack of temporal control over its activity. We have developed a new recombination system using Cre recombinase alpha-complementation. Cre recombinase was divided and one fragment (beta) was introduced into cells between two loxP sites with a CMV promoter in the upstream. The gene of interest (EGFP) was positioned just downstream of this construct. Cre recombinase activity was recovered by adding the other part of the molecule (alpha) to cells as a protein fragment, as evidenced by the expression of EGFP under the control of the CMV promoter. The activity of fragmented cre reached 68% of that of the wild type enzyme at 1 microM alpha-protein.

Conditional gene knockout system in cone photoreceptors

To study function of widely expressed essential genes, we established a conditional knockout system for cone photoreceptor cells. Our goal is to generate a useful genetic system that can be utilized to disrupt gene function efficiently in cone photoreceptor cells. Functional assay using a Cre-activatable lacZ reporter gene suggested that HRGP-cre mice had widely expressed functional Cre in cone photoreceptors. Since nearly all cone photoreceptor cells express Cre in our transgenic mice, they will be efficient in carrying out Cre-mediated gene activation and inactivation in both M- and S-cone photoreceptors. In addition, we have not observed any apparent ectopic expression in HRGP-cre mice (Le et al., 2004); thus, conditional knockout of essential genes with these mice is not likely to cause any detrimental effect in non-ocular tissues.

Engineering embryonic stem cells with recombinase systems

The combined use of site-specific recombination and gene targeting or trapping in embryonic stem cells (ESCs) has resulted in the emergence of technologies that enable the induction of mouse mutations in a prespecified temporal and spatially restricted manner. Their large-scale implementation by several international mouse mutagenesis programs will lead to the assembly of a library of ES cell lines harboring conditional mutations in every single gene of the mouse genome. In anticipation of this unprecedented resource, this chapter will focus on site-specific recombination strategies and issues pertinent to ESCs and mice. The upcoming ESC resource and the increasing sophistication of site-specific recombination technologies will greatly assist the functional annotation of the human genome and the animal modeling of human disease.

Cre-mediated site-specific recombination in zebrafish embryos

Cre-mediated site-specific recombination has become an invaluable tool for manipulation of the murine genome. The ability to conditionally activate gene expression or to generate chromosomal alterations with this same tool would greatly enhance zebrafish genetics. This study demonstrates that the HSP70 promoter can be used to inducibly control expression of an enhanced green fluorescent protein (EGFP) -Cre fusion protein. The EGFP-Cre fusion protein is capable of promoting recombination between lox sites in injected plasmids or in stably inherited transgenes as early as 2 hr post-heat shock induction. Finally, the levels of Cre expression achieved in a transgenic fish line carrying the HSP70-EGFP-cre transgene are compatible with viability and both male and female transgenic fish are fertile subsequent to induction of EGFP-Cre expression. Hence, our data suggests that Cre-mediated recombination is a viable means of manipulating gene expression in zebrafish.

Parenchymal expression of CD86/B7.2 contributes to hepatitis C virus-related liver injury

Hepatitis C virus (HCV) infection is a major global health problem. Hepatic expression of immune costimulatory signaling molecules (e.g., B7) is known to be associated with ongoing liver injury in hepatitis C patients. However, due to the general lack of viral culture systems and adequate animal models, the function of these molecules in disease pathogenesis is poorly understood. To investigate the role of CD86 in HCV-related liver injury, we developed two transgenic mouse lineages with inducible expression of HCV structural proteins and constitutive expression of the costimulatory molecule CD86/B7.2 in the liver. Using a hydrodynamic-based, nonviral delivery protocol, we induced HCV transgene expression in the livers of HCV and CD86 single- and double-transgenic mice. We found that hepatic CD86 expression resulted in increased activation of and cytokine production (e.g., interleukin-2 and gamma interferon) by CD4+ T cells and that the retention of these cells was associated with more pronounced necroinflammatory lesions in the liver. Taken together, these data suggest that augmented, parenchymal antigen presentation conferred by hepatocyte CD86 expression alters homeostasis and effector functions of CD4+ T cells and contributes to liver injury. This study provides an additional rationale for exploring immunomodulation-based therapies that could reduce disease progression in individuals with chronic HCV infection.

Comparative Analysis of Sequence-Specific DNA Recombination Systems in Human Embryonic Stem Cells

The great potential of human embryonic stem cells (hESCs) in basic research, regenerative medicine, and gene therapy is widely recognized. Controlled manipulation of hESC genomes through sequence-specific DNA recombination (SSR) may play a significant role in future hESC applications. However, very little is known about the functionality of SSR systems in hESCs. We demonstrate here that mutant phage lambda integrase, phage P1 Cre recombinase, and mutant gammadelta resolvase displayed distinct activities on episomal recombination substrates. Interestingly, cofactor-independent lambda integrase catalyzed the integrative pathway five times more efficiently than the excisive pathway. Such a degree of directionality in hESCs could be explored for sequential gene insertions into predetermined genomic sequences. We also report an improved, easy-to-use plasmid transfection system that employs silica microspheres and, in combination with SSR, could be applied to hESC genome engineering.

Towards safe, non-viral therapeutic gene expression in humans

The potential dangers of using viruses to deliver and integrate DNA into host cells in gene therapy have been poignantly highlighted in recent clinical trials. Safer, non-viral gene delivery approaches have been largely ignored in the past because of their inefficient delivery and the resulting transient transgene expression. However, recent advances indicate that efficient, long-term gene expression can be achieved by non-viral means. In particular, integration of DNA can be targeted to specific genomic sites without deleterious consequences and it is possible to maintain transgenes as small episomal plasmids or artificial chromosomes. The application of these approaches to human gene therapy is gradually becoming a reality.

Site-specific integration with phiC31 integrase for prolonged expression of therapeutic genes

Need of a site-specific integrating vector in gene therapy has become pressing, as recent work has shown that many of the current integrating vectors used preferentially integrate in the vicinity of genes. A site-specific integrating vector would reduce the risk of insertional mutagenesis posed by randomly integrating vectors, and a non-viral vector would reduce the safety and immunogenicity problems associated with viral vectors. The phiC31 integrase is a protein from Streptomyces phage phiC31 that has been developed as a non-viral site-specific gene therapy vector. The phiC31 integrase catalyzes the integration of a plasmid containing attB into pseudo attP sites in mammalian genomes. It has been shown to function in tissue culture cells as well as in mice. Vectors based on the phiC31 integrase were able to treat tyrosinemia type I in a mouse model and two forms of epidermolysis bullosa in keratinocytes from patients, demonstrating its effectiveness as a gene therapy vector. Development of phiC31 integrase-based vectors is still underway, but it has already been shown to provide long-term expression through site-specific integration.

Enhanced cell-permeant Cre protein for site-specific recombination in cultured cells

Background

Cell-permeant Cre DNA site-specific recombinases provide an easily controlled means to regulate gene structure and function in living cells. Since recombination provides a stable and unambiguous record of protein uptake, the enzyme may also be used for quantitative studies of cis- and trans-acting factors that influence the delivery of proteins into cells.

Results

In the present study, 11 recombinant fusion proteins were analyzed to characterize sequences and conditions that affect protein uptake and/or activity and to develop more active cell-permeant enzymes. We report that the native enzyme has a low, but intrinsic ability to enter cells. The most active Cre proteins tested contained either an N-terminal 6xHis tag and a nuclear localization sequence from SV40 large T antigen (HNC) or the HIV Tat transduction sequence and a C-terminal 6xHis tag (TCH₆). The NLS and 6xHis elements separately enhanced the delivery of the HNC protein into cells; moreover, transduction sequences from fibroblast growth factor 4, HIV Tat or consisting of the (KFF)₃K sequence were not required for efficient protein transduction and adversely affected enzyme solubility. Transduction of the HNC protein required 10 to 15 min for half-maximum uptake, was greatly decreased at 4°C and was inhibited by serum. Efficient recombination was observed in all cell types tested (a T-cell line, NIH3T3, Cos7, murine ES cells, and primary splenocytes), and did not require localization of the enzyme to the nucleus.

Conclusions

The effects of different sequences on the delivery and/or activity of Cre in cultured cells could not be predicted in advance. Consequently, the process of developing more active cell-permeant recombinases was largely empirical. The HNC protein, with an excellent combination of activity, solubility and yield, will enhance the use of cell-permeant Cre proteins to regulate gene structure and function in living cells.

Conditional, recombinase-mediated expression of genes in plant cell cultures

In plant cells, overexpression of critical genes can be hampered by deleterious effects on development that results in a counterselection of transgenic cells harboring the gene of interest. Inducible expression systems have been reported, but many of them show unwanted leaky expression. To circumvent this potential problem, a novel inducible system was developed based on two previously characterized systems: the CRE-loxP site-specific recombination system of bacteriophage P1 and the subcellular targeting of proteins by a mammalian glucocorticoid receptor (GR). By fusing the receptor domain of the rat GR to the carboxyl terminus of the CRE recombinase, a double-lock conditional transcriptional induction system was created that is highly useful to overexpress genes whose expression may block transgenic regeneration. Furthermore, because the designed vector utilizes the GATEWAY recombination technology, cloning was restriction- and ligation-free, thus rendering the vector suitable for high-throughput research. The system was tested in Nicotiana tabacum bright yellow-2 (BY-2) cells and its efficiency was demonstrated for the controlled overexpression of the gus reporter gene and a mutant allele of the A-type cyclin-dependent kinase (CDKA), which is known to be a potent inhibitor of the cell cycle.

α Complementation in the Cre recombinase enzyme

The Cre-loxP system is increasingly exploited for spatial and temporal gene inactivation. Here we present a novel approach to achieve this goal of selective gene inactivation. Following the model of alpha complementation in the beta-galactosidase enzyme, where the enzyme is split into independent polypeptides which are able to associate and maintain the enzymatic activity, we divided the Cre recombinase into two independent polypeptides (one containing the NH(2) terminus (alpha) and a second one containing the COOH-terminus (beta)). Individually, the two polypeptides have no detectable activity. However, when coexpressed the polypeptides are able to associate, giving rise to Cre enzymatic activity, which optimally is as high as 30% of that seen with wildtype Cre recombinase in vitro. We present this strategy as a modification of the traditional Cre-loxP system, which could be used to obtain a highly specific recombination pattern by expressing the two halves under the control of separate promoters.

Analysis of Vir protein translocation from Agrobacterium tumefaciens using Saccharomyces cerevisiae as a model: evidence for transport of a novel effector protein VirE3

Agrobacterium tumefaciens causes crown gall disease on a variety of plants. During the infection process Agrobacterium transfers a nucleoprotein complex, the VirD2 T-complex, and at least two Vir proteins, VirE2 and VirF, into the plant cell via the VirB/VirD4 type IV secretion system. Recently, we found that T-DNA could also be transferred from Agrobacterium to Saccharomyces cerevisiae. Here, we describe a novel method to also detect trans-kingdom Vir protein transfer from Agrobacterium to yeast, using the Cre/lox system. Protein fusions between Cre and VirE2 or VirF were expressed in AGROBACTERIUM: Transfer of the Cre-Vir fusion proteins from Agrobacterium to yeast was monitored by a selectable excision event resulting from site-specific recombination mediated by Cre on a lox-flanked transgene in yeast. The VirE2 and VirF proteins were transported to yeast via the virB-encoded transfer system in the presence of coupling factor VirD4, analogous to translocation into plant cells. The yeast system therefore provides a suitable and fast model system to study basic aspects of trans-kingdom protein transport from Agrobacterium into host cells. Using this method we showed that VirE2 and VirF protein transfer was inhibited by the presence of the Osa protein. Besides, we found evidence for a novel third effector protein, VirE3, which has a similar C-terminal signature to VirE2 and VirF.

Determinants that target the integrase of phage HK022 into the mammalian nucleus

The integrase (Int) protein of coliphage HK022 catalyzes the site-specific integration and excision of the phage into and from its Escherichia coli host chromosome. Int expressed from a plasmid in COS1 monkey cells is localized in the nucleus, as is a fusion protein between Int and the green fluorescent protein (GFP). Mutation analysis of the GFP-Int fusion has revealed in Int two regions of positively charged amino acid residues that cooperate in the nuclear localization. One region harbors residues Arg90 and Arg93. The other, which spans residues 307-340 belongs to the catalytic domain of Int, is rich in basic residues and is strongly conserved within the Int protein family. Being localized in the nucleus renders Int of HK022 as a potential recombinase for site-specific gene manipulations in mammals.

Cre/lox: one more step in the taming of the genome

Cre recombinase has become an important tool in the precise manipulation of the genome, and its adoption has led to the development of increasingly accurate mouse models for the understanding of gene function. Although much of current work exploits the alacrity and precision with which Cre catalyzes excisive DNA recombination, Cre also is adept at the insertion of heterologous DNA into the genome. The precision and efficiency with which Cre can target DNA to a predesignated locus in the genome promises to facilitate understanding of mutant genes and allelic variants in their natural chromosomal context.

Enhanced efficiency through nuclear localization signal fusion on phage PhiC31-integrase: activity comparison with Cre and FLPe recombinase in mammalian cells

The integrase of the phage PhiC31 recombines an attP site in the phage genome with a chromosomal attB site of its Streptomyces host. We have utilized the integrase-mediated reaction to achieve episomal and genomic deletion of a reporter gene in mammalian cells, and provide the first comparison of its efficiency with other recombinases in a new assay system. This assay demonstrated that the efficiency of PhiC31-integrase is significantly enhanced by the C-terminal, but not the N-terminal, addition of a nuclear localization signal and becomes comparable with that of the widely used Cre/loxP system. Furthermore, we found that the improved FLP recombinase, FLPe, exhibits only 10% recombination activity on chromosomal targets as compared with Cre, whereas the Anabaena derived XisA recombinase is essentially inactive in mammalian cells. These results provide the first demonstration that a nuclear localisation signal and its position within a recombinase can be important for its efficiency in mammalian cells and establish the improved PhiC31-integrase as a new tool for genome engineering.

Ability of the hydrophobic FGF and basic TAT peptides to promote cellular uptake of recombinant Cre recombinase: a tool for efficient genetic engineering of mammalian genomes

Conditional mutagenesis is a powerful tool to analyze gene functions in mammalian cells. The site-specific recombinase Cre can be used to recombine loxP-modified alleles under temporal and spatial control. However, the efficient delivery of biologically active Cre recombinase to living cells represents a limiting factor. In this study we compared the potential of a hydrophobic peptide modified from Kaposi fibroblast growth factor with a basic peptide derived from HIV-TAT to promote cellular uptake of recombinant Cre. We present the production and characterization of a Cre protein that enters mammalian cells and subsequently performs recombination with high efficiency in a time- and concentration-dependent manner. Histidine-tagged Cre recombinase transduced inefficiently unless fused to a nuclear localization signal (NLS). Fusion of NLS-Cre to the fibroblast growth factor transduction peptide did not improve the transducibility, whereas fusion with the TAT peptide significantly enhanced cellular uptake and subsequent recombination. More than 95% recombination efficiency in fibroblast cells, as well as murine embryonic stem cells, was achieved after His-TAT-NLS-Cre transduction. Efficient recombination could also be obtained in primary splenocytes ex vivo. We expect that application of His-TAT-NLS-Cre, which can be produced readily in large quantities from a bacterial source, will expand our abilities to manipulate mammalian genomes.

Conditional control of gene expression in the mouse

One of the most powerful tools that the molecular biology revolution has given us is the ability to turn genes on and off at our discretion. In the mouse, this has been accomplished by using binary systems in which gene expression is dependent on the interaction of two components, resulting in either transcriptional transactivation or DNA recombination. During recent years, these systems have been used to analyse complex and multi-staged biological processes, such as embryogenesis and cancer, with unprecedented precision. Here, I review these systems and discuss certain studies that exemplify the advantages and limitations of each system.

Efficient elimination of selectable marker genes from the plastid genome by the CRE-lox site-specific recombination system

Incorporation of a selectable marker gene during transformation is essential to obtain transformed plastids. However, once transformation is accomplished, having the marker gene becomes undesirable. Here we report on adapting the P1 bacteriophage CRE-lox site-specific recombination system for the elimination of marker genes from the plastid genome. The system was tested by the elimination of a negative selectable marker, codA, which is flanked by two directly oriented lox sites (>codA>). Highly efficient elimination of >codA> was triggered by introduction of a nuclear-encoded plastid-targeted CRE by Agrobacterium transformation or via pollen. Excision of >codA> in tissue culture cells was frequently accompanied by a large deletion of a plastid genome segment which includes the tRNA-ValUAC gene. However, the large deletions were absent when cre was introduced by pollination. Thus pollination is our preferred protocol for the introduction of cre. Removal of the >codA> coding region occurred at a dramatic speed, in striking contrast to the slow and gradual build-up of transgenic copies during plastid transformation. The nuclear cre gene could subsequently be removed by segregation in the seed progeny. The modified CRE-lox system described here will be a highly efficient tool to obtain marker-free transplastomic plants.

Conditional gene knockout using Cre recombinase

Cre recombinase has become an important instrument for achieving precise genetic manipulation in mice. Many of these desired genetic manipulations rely on Cre's ability to direct spatially and temporally specified excision of a predesignated DNA sequence that has been flanked by directly repeated copies of the loxP recombination site. Success in achieving such conditional mutagenesis in mice depends both on the careful design of conditional alleles and on reliable detection of cre gene expression. These procedures include PCR, immunohistochemistry and the use of a recombination-proficient GFP-tagged Cre protein.

Inducible and irreversible control of gene expression using a single transgene

Experimental or therapeutic designs involving the conditional expression of genes often require the use of two different transgenes; this can represent a major undertaking. One of these systems takes advantage of inducible recombinases. Here we show a novel use of such enzymes, in that an inducible recombinase-encoding sequence can function to both block the transcription of a gene placed downstream and, subsequently, irreversibly activate transcription of this very same gene. This double function, which circumvents the need for two transgenes, can be achieved by flanking the inducible recombinase gene by two of its target sequences. In our design we used as the inducible recombinase gene the Cre-ER(T) gene flanked by two loxP sites. This cassette was placed between a mouse phosphoglycerate kinase promoter and the enhanced green fluorescent protein (EGFP) coding sequence. Massive EGFP gene expression in BHK cells bearing this transgene was observed upon administration of 4-hydroxytamoxifen (4-OHT), the inducer of the recombinant activity of Cre-ER(T). In the absence of 4-OHT EGFP production was prevented. Because of its simplicity (only a single transgene needs to be used) this strategy is of obvious interest in certain protocols of gene or cell therapy and in a variety of experimental designs in which conditional expression of genes is required.

Site-specific gene targeting for gene expression in eukaryotes

Major advances in the use of site-specific recombinases to facilitate sustained gene expression via chromosomal targeting have been made during the past year. New tools for genomic manipulations using this technology include the discovery of epitopes in recombinases that confer nuclear localization, crystal structures that show the precise topology of recombinase-DNA-substrate synaptic complexes, manipulations of the DNA recognition sequences that select for integration over excision of DNA, and manipulations that make changes in gene expression inducible by drug administration. In addition, endogenous eukaryotic and mammalian DNA sequences have been discovered that can support site-specific recombinase-mediated manipulations.