Transgene Coplacement and High Efficiency Site-Specific Recombination with the Cre/Loxp System in Drosophila

Studies of gene function and regulation in transgenic Drosophila are often compromised by the possibility of genomic position effects on gene expression. We have developed a method, called transgene coplacement, in which any two sequences can be positioned at exactly the same site and orientation in the genome. Transgene coplacement makes use of the bacteriophage P1 system of Cre/loxP site-specific recombination, which we have introduced into Drosophila. In the presence of a cre transgene driven by a dual hsp70-Mos1 promoter, a white reporter gene flanked by loxP sites is excised with virtually 100% efficiency both in somatic cells and in germ cells. A strong maternal effect, resulting from Cre recombinase present in the oocyte, is observed as white or mosaic eye color in F(1) progeny. Excision in germ cells of the F(1) yields a strong grand-maternal effect, observed as a highly skewed ratio of eye-color phenotypes in the F(2) generation. The excision reactions of Cre/loxP and the related FLP/FRT system are used to create Drosophila lines in which transgenes are at exactly allelic sites in homologous chromosomes.     

Highly efficient transgene-independent recombination directed by a maternally derived SOX2CRE transgene

The Cre/loxP site-specific recombination system is a powerful tool that allows gene inactivation in a tissue- and time-specific manner. Several reports have shown that the Sox2Cre transgenic strain provides a very efficient means to delete gene function from the early epiblast (Hayashi et al.: Gene Expr Patterns 2:93-97, 2002; Vincent et al.: Genes Dev 17:1646-1662, 2003). Routinely, male studs carrying one null allele of the gene of interest and the Cre transgene are crossed to females homozygous for the conditional allele. Normally, excision is observed only in the progeny inheriting both the Cre transgene and the conditional allele. Here we report that when the Sox2Cre transgene is inherited maternally, excision occurs in all offspring irrespective of whether they carry the Cre transgene. Thus, Sox2Cre females provide a generally useful tool for rapid and efficient removal of loxP flanked sequences in vivo.     

Efficient in vitro and in vivo excision of floxed sequences with a high-capacity adenoviral vector expressing Cre recombinase

Conditional gene expression or gene disruption using Cre/loxP- or FLP/frt-based recombination systems are valuable tools for studying gene function in development and disease. Recombinant adenoviral vectors expressing Cre recombinase have been suggested as an alternative for deletion of floxed sequences. To further improve this approach we generated a high-capacity adenoviral (HC-Ad) vector expressing Cre (HC-Adcre). In this vector all viral coding sequences are deleted resulting in decreased toxicity. In the present study HC-Adcre efficiently mediated recombination between two loxP sites located in the genome of a reporter cell line. When intravenously injected into ROSA26 reporter mice, a floxed sequence was excised in hepatocytes resulting in expression of the beta-gal reporter. Our data indicate that HC-Ad vectors expressing Cre effectively delete floxed sequences in vivo and have a significant potential as a tool for functional studies in mice.     

Cre重组酶结构与功能的研究进展

Cre/loxP定位重组系统来源于噬菌体P₁,由Cre重组酶和loxP位点两部分组成。在Cre重组酶的介导下,设定的DNA片段可以被切除,可以发生倒位,亦可造成定点的整合。由于其作用方式高效简单,Cre/loxP定位重组系统已在特定基因的删除、基因功能的鉴定、外源基因的整合、基因捕获及染色体工程等方面得到了有效的利用,在转基因的酵母、植物、昆虫、哺乳动物的体内外DNA重组方面成为一个有力的工具。这里就Cre重组酶的结构、功能及该定位重组系统的应用等方面的研究进行了综述。     

A morpholino phenocopy of the cyclops mutation

Summary: Conditional and tissue specific gene targeting using the Cre‐loxP recombination system in combination with established ES cell techniques has become a standard for in vivo loss of function studies. In a typical flox and delete gene targeting strategy, the loxP‐neo‐loxP cassette is inserted into an intron and an additional loxP site is located in one of the homology arms so that loxP sites surround a functionally essential part of the gene. The neo cassette in usually removed by transient expression of the Cre recombinase in ES cells to avoid selection gene interference and genetic ambiquity. However, this causes a significant increase in manipulation of ES cells and often compromises ES cell pluripotency. Here we describe a method in which the floxed neo gene is removed from a knockout allele by infecting 16‐cell‐stage morulae by the recombinant Cre adenovirus. This virus provides only transient Cre expression and does not integrate into the mouse genome. Produced mosaic mice transmitted the desired allele without the neo cassette with high frequency to their offspring. This method is rapid and easy and does not require any special equipment. Moreover, because superovulated mice can be used as donors, this method does not necessitate a large number of mice. genesis 31:126–129, 2001. © 2001 Wiley‐Liss, Inc.     

Cre-mediated recombination in the skin melanocyte lineage

Organ-specific expression of a Cre recombinase allows the analysis of gene function in a particular tissue or cell type. Using a 6.1 kb promoter from the mouse tyrosinase gene, we generated and characterized two lines of transgenic mice that express Cre recombinase in melanoblasts. Utilizing a Cre-responsive reporter mouse strain, genetic recombination was detected in the melanoblasts of the skin from embryonic day 11.5. In addition, Cre-expression was detected in the skin and eyes of mice. Cre transgene activity was occasionally detected in the brain and peripheral nerves but not in other tissues. When Tyr::Cre mice were crossed with mice carrying a homozygous loxP conditional mutation for the insulin-like growth factor receptor gene (Igf1r), Cre-melanoblast-specific recombination pattern was confirmed and no abnormal phenotype was observed. In conclusion, Tyr::Cre transgenic mice provide a valuable tool to follow the cell lineage and to examine gene function in melanocyte development and transformation.     

S Phase-preferential Cre-recombination in mammalian cells revealed by HIV-TAT-PTD-mediated protein transduction

The Cre recombinase of bacteriophage P1 is a powerful tool for artificial modification of genomic function in mammalian cells. To date, many researchers have studied the enzymatic biochemistry of Cre recombinase in loxP site-specific cleavage and rearrangement, as well as its use in gene technology. However, the intricate mechanisms of Cre-mediated recombination are still poorly understood. For example, more knowledge is needed in order to understand Cre recombinase's dependency on cell cycle, the necessity of other factors for recombination, and the exact nuclear environment that's required at the target locus, in order for recombination to take place in eukaryotic cells. In this study, we showed that P1 Cre-mediated recombination occurred frequently during S-phase of the cell cycle. HeLa cells were synchronized in cell cycle with the thymidine-hydroxyurea block method, and recombinant Cre proteins were fused with HIV-1 TAT protein transduction domains (PTD) in every phase of the cell cycle. Results showed that the transduction of PTD-Cre gave rise to genomic recombination preferentially during the S-phase of cell cycle. These findings will contribute significantly to the development of the Cre/loxP recombination system in vivo.     

Genome engineering of Toxoplasma gondii using the site-specific recombinase Cre.

Site-specific DNA recombinases from bacteriophage and yeasts have been developed as novel tools for genome engineering both in prokaryotes and eukaryotes. The 38kDa Cre protein efficiently produces both inter- and intramolecular recombination between specific 34bp sites called loxP. We report here the in vivo use of Cre recombinase to manipulate the genome of the protozoan parasite Toxoplasma gondii. Cre catalyzes the precise removal of transgenes from T. gondii genome when flanked by two directly repeated loxP sites. The efficiency of excision has been determined using LacZ as reporter and indicates that it can easily be applied to the removal of undesired sequences such as selectable marker genes and to the determination of gene essentiality. We have also shown that the reversibility of the recombination reaction catalyzed by Cre offers the possibility to target site-specific integration of a loxP-containing vector in a chromosomally placed loxP target in the parasite. In mammalian systems, the Cre recombinase can be regulated by hormone and is used for inducible gene targeting. In T. gondii, fusions between Cre recombinase and the hormone-binding domain of steroids are constitutively active, hampering the utilization of this mode of post-translational regulation as inducible gene expression system.     

Simultaneous on/off regulation of transgenes located on a mammalian chromosome with Cre-expressing adenovirus and a mutant loxP

The site-specific recombinase Cre has often been used for on/off regulation of expression of transgenes introduced into the mammalian chromosome. However, this method is only applicable to the regulation of a single gene and cannot be used to simultaneously regulate two genes, because site-specific recombination occurs from the target loxP sequence of one regulating unit to the loxP sequence of any other unit and would eventually disrupt the structure of both regulating units. We previously reported a mutant loxP sequence with a two base substitution called loxP V (previously called loxP 2272), with which wild-type loxP cannot recombine but with which the identical mutant loxP recombines efficiently. In the present study we isolated cell lines bearing two regulating units on a chromosome containing a pair of wild-type loxP sequences or mutant loxP V sequences. After infection with Cre-expressing recombinant adenovirus AxCANCre, expression of a gene in each regulating unit was simultaneously turned on and off. Southern analyses showed that both regulating units were processed simultaneously and independently, even after infection with a limited amount of AxCANCre. The results showed that simultaneous regulation of gene expression on a mammalian chromosome mediated by Cre can be achieved by using mutant loxP V and wild-type loxP. This method may be a useful approach for conditional transgenic/knockout animals and investigation of gene function involving two genes simultaneously. Another possible application is for preparation of a new packaging cell line of viral vectors through simultaneous production of toxic viral genes.     

Induced DNA recombination by Cre recombinase protein transduction

Cre is a DNA recombinase that recognizes 34 base-pair loxP sites of recombination. We have developed a cell-permeable Cre recombinase, TATCre, that is capable of mediating deletion of loxP-flanked targets by simply adding TATCre to cell cultures. Thus, TATCre allows efficient induced DNA recombination without the use of a Cre recombinase transgene or any other genetic material and should prove useful for the genetic manipulation of a wide variety of cell types that have been engineered to possess loxP sites.     

Development of a novel helper-dependent adenovirus-Epstein-Barr virus hybrid system for the stable transformation of mammalian cells

Epstein-Barr virus (EBV) episomes are stably maintained in permissive proliferating cell lines due to EBV nuclear antigen 1 (EBNA-1) protein-mediated replication and segregation. Previous studies showed the ability of EBV episomes to confer long-term transgene expression and correct genetic defects in deficient cells. To achieve quantitative delivery of EBV episomes in vitro and in vivo, we developed a binary helper-dependent adenovirus (HDA)-EBV hybrid system that consists of one HDA vector for the expression of Cre recombinase and a second HDA vector that contains all of the sequences for the EBV episome flanked by loxP sites. Upon coinfection of cells, Cre expressed from the first vector recombined loxP sites on the second vector. The resulting circular EBV episomes expressed a transgene and contained the EBV-derived family of repeats, an EBNA-1 expression cassette, and 19 kb of human DNA that functions as a replication origin in mammalian cells. This HDA-EBV hybrid system transformed 40% of cultured cells. Transgene expression in proliferating cells was observed for over 20 weeks under conditions that selected for the expression of the transgene. In the absence of selection, EBV episomes were lost at a rate of 8 to 10% per cell division. Successful delivery of EBV episomes in vivo was demonstrated in the liver of transgenic mice expressing Cre from the albumin promoter. This novel gene transfer system has the potential to confer long-term episomal transgene expression and therefore to correct genetic defects with reduced vector-related toxicity and without insertional mutagenesis.     

角质细胞特异性表达Cre重组酶转基因小鼠的建立

构建了含有角质细胞特异性角质素5启动子、Cre重组酶基因和人生长激素基因plyA的转基因载体pK5-Cre-hGH。以显微注射的方法将4．2kb的转基因片段K5-Cre-hGH引入小鼠基因组,共注射720枚受精卵,其中龄5枚移植至29只假孕母鼠的输卵管中发育,获得子代小鼠48只,经基因型鉴定有12只小鼠在其基因组上整合有Cre基因,整合率为25％。将带有cre重组酶基因的小鼠与基因组上携带loxP位点的smad4条件基因打靶小鼠杂交以检测Cre重组酶组织特异性表达情况以及介导重组的功能。结果表明,K5-Cre转基因小鼠只在皮肤组织中表达Cre重组酶并能在体内成功地介导loxP位点的重组。     

Spontaneous ectopic recombination in cell-type-specific Cre mice removes loxP-flanked marker cassettes in vivo

Conditional gene targeting using the Cre/loxP technology generally includes integration of a selection marker cassette flanked by loxP recognition sites (floxed) in the target gene locus. Subsequent marker removal avoids possible impairment of gene expression or mosaicism due to partial and total deletions after Cre-mediated recombination in vivo. The use of deleter Cre mice for in vivo marker removal in floxed connexin43 mice revealed considerable mosaicism, but no selective marker removal. In addition, we noted that several Cre transgenic lines displayed spontaneous ectopic activity, reminiscent of deleter Cre mice, and required the confirmation of cell type-specific deletion in every individual mouse. When we used myosin heavy chain promoter Cre (alphaMyHC-Cre) mice for cardiomyocyte specific deletion, we observed, in addition to cardiomyocyte-restricted or complete excision, selective marker removal in a subgroup of mice as well. Thus, selective marker removal can be achieved as a byproduct of cell-type restricted deletion.     

利用不相容质粒共转化大肠杆菌对Cre重组酶体内重组活性的可视检测

源自噬菌体P1的Cre重组酶可以识别 34bp的靶DNA序列loxP ,进行位点特异性的重组反应。为了简便地检测Cre酶在大肠杆菌中的重组活性 ,分别将cre基因和上下游带有loxP的绿色荧光蛋白基因 (gfp)克隆到具有不同抗性的两种不相容质粒中 ,然后将构建的原核表达载体pET30a Cre和pET2 3b loxGFP电击共转化大肠杆菌BL2 1(DE3) ,利用卡那霉素和氨苄青霉素双抗生素抗性进行筛选。通过直接观察转化子的绿色荧光 ,便可以显示Cre酶的体内重组活性 ,并进一步通过SDS PAGE分析、质粒酶切鉴定进行了验证。结果表明 :以gfp为报告基因、通过两种不相容质粒共转化大肠杆菌可以为研究和改进Cre loxP重组系统提供一种简便直观的检测方法     

A keratin K5Cre transgenic line appropriate for tissue-specific or generalized Cre-mediated recombination

We describe here a mouse line bearing a bovine keratin K5Cre recombinase transgene. These mice showed a dual pattern of Cre-mediated recombination, depending on the parent transmitting the transgene. In paternal transmission, recombination occurred specifically in the skin and stratified epithelia-as expected according to the expression of endogenous keratin K5. However, constitutive recombination between loxP sites transmitted by the sperm took place when the mother possessed the K5Cre transgene, even when the transgene was absent in the progeny. Cre expression in late-stage oocytes, with the Cre protein persisting into the developing embryo, leads to the constitutive recombination observed. Thus, this transgenic line allows for both tissue-specific and generalized recombination, depending on the breeding scheme.     

Conditional transgene expression mediated by the mouse beta-actin locus

Transgenic mice are an effective model to study gene function in vivo; however, position effects can complicate tissue-specific transgene analysis. To facilitate precise targeting of a transgenic construct into the mouse genome, we combined the Cre/lox and Flp/FRT recombination systems to allow for rapid transgene replacement and conditional transgene expression from the endogenous beta-actin locus. Flp/FRT recombination was used to rapidly exchange FRT-flanked transgene cassettes by recombinase-mediated cassette exchange in embryonic stem cells, while transgene expression can be activated in mice after Cre-mediated excision of a floxed STOP cassette. To validate our system, we analyzed the expression profile of an EGFP reporter gene after integration into the beta-actin locus and Cre-mediated excision of the floxed STOP cassette. Breeding of EGFP reporter mice with various Cre mouse lines resulted in the expected expression profiles, demonstrating the feasibility of the model to facilitate predictable and strong transgene expression in a spatially and temporally controlled manner.     

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.