Split-Cre Complementation Restores Combination Activity on Transgene Excision in Hair Roots of Transgenic Tobacco

The Cre/loxP system is increasingly exploited for genetic manipulation of DNA in vitro and in vivo. It was previously reported that inactive ‘‘split-Cre’’ fragments could restore Cre activity in transgenic mice when overlapping co-expression was controlled by two different promoters. In this study, we analyzed recombination activities of split-Cre proteins, and found that no recombinase activity was detected in the in vitro recombination reaction in which only the N-terminal domain (NCre) of split-Cre protein was expressed, whereas recombination activity was obtained when the C-terminal (CCre) or both NCre and CCre fragments were supplied. We have also determined the recombination efficiency of split-Cre proteins which were co-expressed in hair roots of transgenic tobacco. No Cre recombination event was observed in hair roots of transgenic tobacco when the NCre or CCre genes were expressed alone. In contrast, an efficient recombination event was found in transgenic hairy roots co-expressing both inactive split-Cre genes. Moreover, the restored recombination efficiency of split-Cre proteins fused with the nuclear localization sequence (NLS) was higher than that of intact Cre in transgenic lines. Thus, DNA recombination mediated by split-Cre proteins provides an alternative method for spatial and temporal regulation of gene expression in transgenic plants.     

Ssp DnaE split-intein mediated split-Cre reconstitution in tobacco

The Cre/loxP system is increasingly exploited for spatial and temporal gene activation or inactivation. In this study, a novel approach for gene activation using a Cre/loxp system in tobacco is described. As the DnaE intein in Synechocystis sp. strain PCC6803 is capable of catalyzing a protein trans-splicing reaction to assemble a mature protein from two separate precursors, the N- and C-terminal ends of the Cre enzyme, split between Gly190 and Gly191, were fused to N- and C-terminals of the Ssp DnaE split intein,respectively. Subsequently, in-frame fusions of NCre/NInt and CInt/CCre are assembled into the pCAMBIA1300 cloning vector, and used for co-expression, along with the BAR selectable marker gene for BASTA herbicide resistance in tobacco. A Cre-dependent excision recombination event is monitored when tobacco leaf explants are screened for resistance to Basta, but along with absence of beta-glucuronidase activity. Based on herbicide resistance, an efficient recombination event is observed, in vivo Bar activation following co-expression of NCre/NInt and CInt/CCre fusion genes in pCAGUS/BAR transgenic lines. Moreover, the recombination efficiency is comparable to that of intact Cre gene expression. However, no Cre recombination event is observed when only the NCre and CCre genes or the NCre/NInt fusion gene and the CCre genes are co-expressed. Thus, the Ssp DnaE split intein-mediated Cre activity reconstitution observed in this study provides an alternative approach for the traditional Cre/loxP system, and this may aid in achieving dynamic regulation of gene expression in transgenic plants.     

Brief expression of a GFP cre fusion gene in embryonic stem cells allows rapid retrieval of site-specific genomic deletions.

The Cre DNA recombinase of bacteriophage P1 has become a useful tool for precise genomic manipulation in embryonic stem (ES) cells that have been gene modified by homologous recombination. We have re-engineered the cre gene to allow ready identification of living Cre+cells by constructing a functional fusion between Cre and an enhanced green fluorescent protein from Aequorea victoria (GFPS65T). The GFP cre fusion gene product rapidly targeted the nucleus in the absence of any exogenous nuclear localization signal. Moreover, GFPCre catalyzed efficient DNA recombination in both a mouse 3T3 derivative cell line and in murine ES cells. Fluorescence- activated cell sorting (FACS) of transiently GFP cre -transfected ES cells not only allowed rapid and efficient isolation of Cre+cells after DNA transfection but also demonstrated that a burst of Cre expression is sufficient to commit cells to Cre-mediated 'pop-out' of loxP -tagged DNA from the genome. Thus, GFP cre allows rapid identification of living cells in which loxP - flanked DNA sequences are destined to be removed from the genome by Cre-mediated recombination without reliance on recombinational activation or inactivation of a marker gene at the target locus. In addition, the GFP cre fusion gene will prove useful in tracing tissue-specific Cre expression in transgenic animals, thereby facilitating the generation and analysis of conditional gene knockout mice.     

Generation of chickens expressing Cre recombinase

Cre recombinase has been extensively used for genome engineering in transgenic mice yet its use in other species has been more limited. Here we describe the generation of transgenic chickens expressing Cre recombinase. Green fluorescent protein (GFP)-positive chicken primordial germ cells were stably transfected with β-actin-Cre-recombinase using phiC31 integrase and transgenic chickens were generated. Cre recombinase activity was verified by mating Cre birds to birds carrying a floxed transgene. Floxed sequences were only excised in offspring from roosters that inherited the Cre recombinase but were excised in all offspring from hens carrying the Cre recombinase irrespective of the presence of the Cre transgene. The Cre recombinase transgenic birds were healthy and reproductively normal. The Cre and GFP genes in two of the lines were closely linked whereas the genes segregated independently in a third line. These founders allowed development of GFP-expressing and non-GFP-expressing Cre recombinase lines. These lines of birds create a myriad of opportunities to study developmentally-regulated and tissue-specific expression of transgenes in chickens.     

Use of green fluorescent protein/Flp recombinase fusion protein and flow cytometric sorting to enrich for cells undergoing Flp-mediated recombination

Flp recombinase has been used extensively for in vivo manipulation of eukaryotic DNA at specific sequences designated as FRT sites. We developed a method to use Flp-mediated recombination without the need for drug resistance or metabolic selection of cells in which recombination has occurred. We generated expression plasmids directing expression of fusion proteins consisting of Flp recombinase and green fluorescent protein (GFP) coding sequences. When the plasmids were introduced into K562 cells containing Flp recombinase substrates and transfected cells were selected for by flow cytometric sorting, GFP-positive cells were enriched 5- to 30-fold for Flp-mediated recombination events compared with unsorted cells. These studies demonstrate the usefulness of GFP/Flp recombinase fusion proteins to manipulate chromosomal DNA in vivo without requiring drug resistance or metabolic marker genes.     

Comparison of the tamoxifen regulated chimeric Cre recombinases MerCreMer and CreMer

The site-directed recombinase Cre can be employed to delete or assemble genes in the genome of living cells. We have constructed expression vectors for chimeric Cre recombinases carrying a mutated hormone binding domain either at the C-terminus only (CreMer) or at both the N and C-termini (MerCreMer). The chimeric Cre proteins can be activated by culturing transfected cells with 4-hydroxytamoxifen. In transfected embryonic stem (ES) cells, we compared the extent of recombination of a floxed gene with the expression levels of the chimeric Cre proteins. Our data demonstrate that the bulky MerCreMer protein is not less active than the CreMer protein. Thus, the tighter control and low background activity of the MerCreMer enzyme is not due to a generally low recombinase efficiency.     

Enhancing the stability of trehalose synthase via SpyTag/SpyCatcher cyclization to improve its performance in industrial biocatalysts

SpyTag and SpyCatcher can spontaneously and rapidly conjugate to form an irreversible and stable covalent bond. The trehalose synthase (TreS) from Thermomonospora curvata was successfully cyclized after the fusion of a SpyTag to its C-terminus and SpyCatcher to the N-terminus. Cyclized TreS retained more than 85% of its activity at temperatures ranging from 40 to 50°C and more than 95% at a pH range of 8 to 10, while the wild type kept only 60 and 80% of its activity under the same conditions. These results demonstrated that cyclized TreS had better resistance to high temperature and alkali than the wild type. Furthermore, structural analysis revealed that cyclized TreS had better conformational stability and was able to fold correctly at a higher temperature than the wild type. Our findings indicate that the use of SpyTag and SpyCatcher to cyclize enzymes is a promising strategy to increase their stability.     

Regulation of Cre recombinase by ligand-induced complementation of inactive fragments

Cre recombinase is extensively used to engineer the genome of experimental animals. However, its usefulness is still limited by the lack of an efficient temporal control over its activity. To overcome this, we have developed DiCre, a regulatable fragment complementation system for Cre. The enzyme was split into two moieties that were fused to FKBP12 (FK506-binding protein) and FRB (binding domain of the FKBP12–rapamycin-associated protein), respectively. These can be efficiently heterodimerized by rapamycin. Several variants, based on splitting Cre at different sites and using different linker peptides, were tested in an indicator cell line. The fusion proteins, taken separately, had no recombinase activity. Stable transformants, co-expressing complementing fragments based on splitting Cre between Asn59 and Asn60, displayed low background activity affecting 0.05–0.4% of the cells. Rapamycin induced a rapid recombination, reaching 100% by 48–72 h, with an EC₅₀ of 0.02 nM. Thus, ligand-induced dimerization can efficiently regulate Cre, and should be useful to achieve a tight temporal control of its activity, such as in the case of the creation of conditional knock-out animals.     

Inducible site-directed recombination in mouse embryonic stem cells.

The site-directed recombinase Cre can be employed to delete or express genes in cell lines or animals. Clearly, the ability to control remotely the activity of this enzyme would be highly desirable. To this end we have constructed expression vectors for fusion proteins consisting of the Cre recombinase and a mutated hormone-binding domain of the murine oestrogen receptor. The latter still binds the anti-oestrogen drug tamoxifen but no longer 17 beta-oestradiol. We show here that in embryonic stem cells expressing such fusion proteins, tamoxifen can efficiently induce Cre-mediated recombination, thereby activating a stably integrated LacZ reporter gene. In the presence of either 10 microM tamoxifen or 800 nM 4-hydroxy-tamoxifen, recombination of the LacZ gene is complete within 3-4 days. By placing a tamoxifen-binding domain on both ends of the Cre protein, the enzymatic activity of Cre can be even more tightly controlled. Transgenic mice expressing such an tamoxifen-inducible Cre enzyme may thus provide a new and useful genetic tool to mutate or delete genes at specific times during development or in adult animals.     

Cre‐mediated transgene activation in the developing and adult mouse brain

Summary: The neuron‐specific rat enolase (NSE) promoter was employed to establish transgenic mice expressing Cre recombinase in the central nervous system. Founders were crossed with dormant lacZ indicator mice and specificity as well as efficiency of Cre‐mediated transgene activation was determined by PCR and/or X‐gal staining. Whereas most transgenic lines exhibited Cre activity in early development resulting in widespread Cre activity, one line (NSE‐Cre26) expressed high levels of Cre in the developing and adult brain. With the exception of kidney, which showed occasionally low level of Cre activity, Cre recombination in double transgenics was restricted to the nervous system. Whole‐mount X‐gal staining of 9.5 dpc embryos indicated Cre‐mediated lacZ expression in forebrain, hindbrain, and along the midbrain flexure. A similar expression pattern was observed during later stages of embryogenesis (11.5–13.5 dpc). In adult mice, Cre recombinase was expressed in cerebral cortex and cerebellum and high levels of Cre‐mediated lacZ expression were observed in hippocampus, cortex, and septum. The NSE‐Cre26 transgenic mouse line thus provides a useful tool to specifically overexpress and/or inactivate genes in the developing and adult brain. genesis 31:118–125, 2001. © 2001 Wiley‐Liss, Inc.     

Structural Analysis and Optimization of the Covalent Association between SpyCatcher and a Peptide Tag

Peptide tagging is a key strategy for observing and isolating proteins. However, the interactions of proteins with peptides are nearly all rapidly reversible. Proteins tagged with the peptide SpyTag form an irreversible covalent bond to the SpyCatcher protein via a spontaneous isopeptide linkage, thereby offering a genetically encoded way to create peptide interactions that resist force and harsh conditions. Here, we determined the crystal structure of the reconstituted covalent complex of SpyTag and SpyCatcher at 2.1 Å resolution. The structure showed the expected reformation of the β-sandwich domain seen in the parental streptococcal adhesin, but flanking sequences at both N- and C-termini of SpyCatcher were disordered. In addition, only 10 out of 13 amino acids of the SpyTag peptide were observed to interact with SpyCatcher, pointing to specific contacts important for rapid split protein reconstitution. Based on these structural insights, we expressed a range of SpyCatcher variants and identified a minimized SpyCatcher, 32 residues shorter, that maintained rapid reaction with SpyTag. Together, these results give insight into split protein β-strand complementation and enhance a distinct approach to ultrastable molecular interaction.     

New approach to cell lineage analysis in mammals using the Cre-loxP system

The Cre-loxP site-specific recombination system was used for cell lineage analysis in mammals. We constructed an expression plasmid, pCETZ-17, which consists of cytomegalovirus enhancer/chicken beta-actin promoter (CAG), a portion of the rabbit beta-globin gene, loxP-flanked DNA sequence (containing enhanced green fluorescent protein (EGFP) cDNA), and lacZ gene encoding E. coli beta-galactosidase (beta-gal). When circular pCETZ-17 plasmid DNA was microinjected into the pronuclei of fertilized eggs and these eggs were allowed to develop to two-cell stage, 62.8% (59/94) of the two-cell embryos exhibited distinct fluorescence in one or both blastomeres, but never expressed lacZ protein, as evaluated by histochemical staining with X-Gal, a substrate for beta-gal. When both circular plasmids, pCETZ-17 and pCAG/NCre (containing CAG and DNA sequences encoding nuclear location signal and Cre), were co-injected into fertilized eggs, almost all (87.0%, 47/54) embryos exhibited low or no fluorescence, but 51.9% (27/52) exhibited positive staining for beta-gal activity. This indicates that transient expression of the Cre recombinase gene removed the loxP-flanked DNA sequence in pCETZ-17 and then caused expression of the downstream lacZ sequence. We next microinjected pCETZ-17 into the pronuclei of fertilized eggs, cultured these injected eggs for 1 day, and collected only two-cell embryos expressing EGFP in both blastomeres. One blastomere of the EGFP-expressing two-cell embryos was microinjected with pCAG/NCre, and these treated embryos were cultured for 1 day up to four-cell stage. When the developing four-cell embryos were subjected to staining with X-Gal, cell lineage-related staining pattern for beta-gal activity was observed in most (77.8%, 7/9) embryos. These findings were further confirmed using two-cell embryos derived from a transgenic mouse line carrying CETZ-17 transgene. Thus, our system, which is based on transient expression of the Cre recombinase gene directly introduced into nuclei of embryonic cells by microinjection, is a powerful means for cell lineage analysis in mammals.     

Unmodified Cre recombinase crosses the membrane

Site-specific recombination in genetically modified cells can be achieved by the activity of Cre recombinase from bacteriophage P1. Commonly an expression vector encoding Cre is introduced into cells; however, this can lead to undesired side-effects. Therefore, we tested whether cell-permeable Cre fusion proteins can be directly used for lox-specific recombination in a cell line tailored to shift from red to green fluorescence after loxP-specific recombination. Comparison of purified recombinant Cre proteins with and without a heterologous ‘protein transduction domain’ surprisingly showed that the unmodified Cre recombinase already possesses an intrinsic ability to cross the membrane border. Addition of purified recombinant Cre enyzme to primary bone marrow cells isolated from transgenic C/EBPα^fl/fl mice also led to excision of the ‘floxed’ C/EBPα gene, thus demonstrating its potential for in vivo applications. We conclude that Cre enyzme itself or its intrinsic membrane-permeating moiety are attractive tools for direct manipulation of mammalian cells.     

Preparation of enzymatically active human Myc-tagged-NCre recombinase exhibiting immunoreactivity with anti-Myc antibody

The Cre-loxP system has been recognized as a tool for conditional gene targeting in mice. However, most anti-Cre antibodies fail to react with Cre expressed in vivo. In an attempt to directly detect Cre by antibodies in vivo, we constructed the tagged-NCre (NCreMH) gene by connecting the human Myc and His tag sequences to the 3' end of the NCre gene carrying a nuclear localizing signal (NLS) sequence. The production of NCre protein and the recombinase activity were detected after co-transfection with pCMV-NCreMH and pCETZ-17 carrying the loxP-flanked lacZ gene into NIH3T3 cells. This activity was also confirmed in vivo after gene transfer of pCMV-NCreMH and pCRTEIL-6 carrying loxP-flanked HcRed1 and EGFP cDNAs, into oviductal epithelium by electroporation. Immunohistochemical staining using anti-Myc antibody demonstrated that the area positive for enhanced green fluorescent protein (EGFP) fluorescence was immunostained with the antibody. These findings indicate that NCreMH is useful as an alternative to NCre for gene targeting.     

Reduction of Cre recombinase toxicity in proliferating Drosophila cells by estrogen-dependent activity regulation

The Cre/loxP site-specific recombination system has been used successfully for genome manipulation in a wide range of species. However, in Drosophila melanogaster, a major model organism for genetic analyses, the alternative FLP/FRT system, which is less efficient at least in mammalian cells, has been established, primarily for the generation of genetic mosaics for clonal analyses. To extend genetic methodology in D. melanogaster, we have created transgenic lines allowing tissue-specific expression of Cre recombinase with the UAS/GAL4 system. Surprisingly, chronic expression of Cre recombinase from these transgenes (UAST-cre) was found to be toxic for proliferating cells. Therefore, we also generated transgenic lines allowing the expression of Cre recombinase fused to the ligand-binding domain of the human estrogen receptor (UASP-cre-EBD). We demonstrate that recombination can be efficiently dissociated from toxicity by estrogen-dependent regulation of recombinase activity of the UASP-cre-EBD transgene products.     

Flp recombinase transgenic mice of C57BL/6 strain for conditional gene targeting

We constructed an expression vector of Flp recombinase modified by adding a nuclear localization signal. Injection of the expression vector into fertilized eggs of the C57BL/6 strain yielded transgenic mouse lines expressing the Flp recombinase transgene in the testis. We crossed the transgenic mice to reporter mice carrying the neomycin phosphotransferase gene flanked by target sites of Flp recombinase. Examination of the deletion of the neomycin phosphotransferase gene in the progeny showed that Flp-mediated recombination took place efficiently in vivo in FLP66 transgenic mouse line. These results suggest that the Flp recombinase system is effective in mice and in combination with the Cre recombinase system extends the potentials of gene manipulation in mice. One of the useful applications of FLP66 transgenic mouse line is the removal of marker genes from mice manipulated for the conditional gene targeting with the Cre/loxP system in the pure C57BL/6 genetic background.     

Codon-improved Cre recombinase (iCre) expression in the mouse

By applying the mammalian codon usage to Cre recombinase, we improved Cre expression, as determined by immunoblot and functional analysis, in three different mammalian cell lines. The improved Cre (iCre) gene was also designed to reduce the high CpG content of the prokaryotic coding sequence, thereby reducing the chances of epigenetic silencing in mammals. Transgenic iCre expressing mice were obtained with good frequency, and in these mice loxP-mediated DNA recombination was observed in all cells expressing iCre. Moreover, iCre fused to two estrogen receptor hormone binding domains for temporal control of Cre activity could also be expressed in transgenic mice. However, Cre induction after administration of tamoxifen yielded only low Cre activity. Thus, whereas efficient activation of Cre fusion proteins in the brain needs further improvements, our studies indicate that iCre should facilitate genetic experiments in the mouse.     

Nuclear targeting determinants of the phage P1 cre DNA recombinase

The Cre DNA recombinase of bacteriophage P1 has become a useful tool for genomic manipulation in mice and other eukaryotes. Because Cre is of prokaryotic origin, the 38 kDa protein has been presumed to gain access to the eukaryotic nucleus simply because it is sufficiently small to pass through the nuclear pore by passive diffusion. Instead, we show here that Cre carries nuclear targeting determinants that efficiently direct Cre entry into the nucleus of mammalian cells. Fusions of Cre with green fluorescent protein (GFP) identified two regions that are necessary for nuclear localization. Region I contains a cluster of basic amino acids that is essential for nuclear localization and which resembles a bipartite-like nuclear localization signal. Region II exhibits a beta-sheet structure with which the bipartite motif may interact. However, neither region is by itself sufficient for nuclear localization. Nuclear transport in vitro with a 98 kDa GFP-Cre fusion protein shows that Cre does not gain access to the nucleus by passive diffusion, but instead enters the nucleus by means of an energy-dependent process. Thus, Cre is one of the few prokaryotic proteins that have been shown to carry determinants that allow it to target the eukaryotic nucleus.