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Ability of the Hydrophobic FGF and Basic TAT Peptides To 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 Michael Peitz, Kurt Pfannkuche, Klaus Rajewsky*, and Frank Edenhofer† Institute for Genetics, University of Cologne, Weyertal 121, 50931 Cologne, Germany Contributed by Klaus Rajewsky, February 5, 2002 Conditional mutagenesis is a powerful tool to analyze gene func- sive mouse breeding causing the experiments to be time con- tions in mammalian cells. The site-specific recombinase Cre can be suming and costly. The leakiness of the system represents a used to recombine loxP-modified alleles under temporal and spa- critical factor because a Cre recombinase that is undesirably tial control. However, the efficient delivery of biologically active active before induction often leads to unwanted side effects such Cre recombinase to living cells represents a limiting factor. In this as mosaic recombination and͞or selection of recombined or study we compared the potential of a hydrophobic peptide mod- nonrecombined cells both in vivo and in vitro (9, 17, 18). ified from Kaposi fibroblast growth factor with a basic peptide Moreover, the widely used inducers IFN, hydroxy-tamoxifen, derived from HIV-TAT to promote cellular uptake of recombinant and doxycycline are known to display toxic side effects (19, 20) Cre. We present the production and characterization of a Cre and͞or induce also unwanted physiological effects that may protein that enters mammalian cells and subsequently performs interfere with the experimental phenotype of the conditional recombination with high efficiency in a time- and concentration- mutation to be analyzed (21, 22). In cultured cells, Cre-mediated dependent manner. Histidine-tagged Cre recombinase transduced recombination is limited by transfection efficiencies and putative inefficiently unless fused to a nuclear localization signal (NLS). toxicity of the protein (13, 23, 24). Thus, traditional delivery of Fusion of NLS-Cre to the fibroblast growth factor transduction Cre—either by Cre transgenics, viral vectors, or transfection— peptide did not improve the transducibility, whereas fusion with represents a limiting step of conditional mutagenesis employing the TAT peptide significantly enhanced cellular uptake and subse- ͞ quent recombination. More than 95% recombination efficiency in Cre loxP technology. fibroblast cells, as well as murine embryonic stem cells, was Protein transduction is a recently developed method to intro- achieved after His-TAT-NLS-Cre transduction. Efficient recombina- duce biologically active proteins directly into mammalian cells tion could also be obtained in primary splenocytes ex vivo.We with high efficiency (for review, see refs. 25 and 26). Recombi- expect that application of His-TAT-NLS-Cre, which can be produced nant technology has been used to modify biophysical properties readily in large quantities from a bacterial source, will expand our of proteins, particularly with respect to their cell permeability, by abilities to manipulate mammalian genomes. employing so-called protein transduction domains (PTDs) (27– 29). It has been demonstrated that a basic 11-aa peptide, derived from HIV-TAT, renders ␤-galactosidase (␤-gal) into a cell- onditional mutagenesis in mammalian cells has become an GENETICS permeable form. ␤-gal activity can be detected in organs such as important means for the analysis of gene function in vivo (for C liver, kidney, lung, brain, and spleen of mice after i.p. injection review, see refs. 1–3). Site-specific recombinases such as the ␤ bacteriophage P1 recombinase Cre have been used to gain of TAT- -gal (28). Besides the basic TAT peptide, other pep- control over the mutation in a spatial (4–6) and͞or temporal tides or protein (fragments) also have been reported to enhance manner (7, 8). An increasing number of studies have demon- cellular uptake of proteins, such as a hydrophobic peptide strated the efficacy of Cre-mediated conditional mutagenesis in modified from Kaposi fibroblast growth factor (FGF) (29, 30). mice and cell lines (9–12). Usually a mouse or cell line is Recently it has been reported that a fusion protein of a nuclear generated, in which an essential part of the gene of interest is localization signal (NLS), Cre recombinase, and a FGF peptide flanked by two loxP sites. The loxP sites represent Cre recom- displays cell permeability. After direct application of the recom- binase recognition sites and can be used to delete the respective binant protein to cultured cells ofaTlymphocyte line containing gene segment upon Cre recombination, resulting in a condi- a loxP-modified substrate, Cre-mediated recombination was tioned inactivation or mutation of the gene of interest. To gain observed in Ϸ80% of the cells (31). However, it remained temporal control over this mutation event two different ap- unclear from this study whether the FGF peptide is essential for proaches have been applied: (i) Cre is delivered into cultured transduction or even contributes significantly to the cellular cells either by transfection (13) or adenoviral infection (14, 15); uptake of Cre. (ii) Cre recombinase activity is induced by application of an exogenous inducer. Induction can be carried out either at the transcriptional level [e.g., Mx-Cre (7) or tetracycline-controlled Abbreviations: aa, amino acid(s); ␤-gal, ␤-galactosidase; ES, embryonic stem cell(s); FGF, fibroblast growth factor; NLS, nuclear localization signal; PTD, protein transduction Cre expression (16)] or posttranslational level employing fusion domain. proteins of Cre with mutated ligand-binding domains of steroid *Present address: Center for Blood Research, Harvard Medical School, 200 Longwood receptors (8). Although a number of studies demonstrated the Avenue, Boston, MA 02115. efficacy of Cre-mediated inducible mutagenesis, still the system †To whom reprint requests should be addressed. E-mail: [email protected]. could profit substantially from technical advance concerning The publication costs of this article were defrayed in part by page charge payment. This three major aspects: leakiness of the system before induction, article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. efficiency of induced recombination, and requirement of exten- §1734 solely to indicate this fact. www.pnas.org͞cgi͞doi͞10.1073͞pnas.032068699 PNAS ͉ April 2, 2002 ͉ vol. 99 ͉ no. 7 ͉ 4489–4494 Downloaded by guest on October 2, 2021 Our study was aimed at evaluating the actual potency of two Overexpression was induced by adding IPTG to a final concen- prominent PTDs, namely FGF and TAT peptides, to promote tration of 0.5 mM and an additional incubation of 3–4hat37°C. the translocation of biologically active Cre across the plasma Cells were harvested by centrifugation and frozen in dry ice͞ membrane of mammalian cells. We generated expression vectors ethanol and stored at Ϫ20°C. Cell pellets were thawed by ͞ encoding Cre recombinase fused to FGF and TAT peptides, resuspending in lysis buffer [100 mM NaH2PO4 10 mM Tris (pH respectively. The potentials of the recombinant proteins to 8.0)͞300 mM NaCl͞10 mM imidazole] at 5 ml per gram wet transduce and subsequently recombine loxP-flanked targets in weight. Cleared lysate was obtained after incubation with 2 mammalian cells were compared side by side with Cre lacking mg͞ml lysozyme (Sigma) and benzonase (Novagen) and centrif- any particular PTD. ugation for 25 min at 30,000 ϫ g at 4°C. One milliliter of 50% Ni-NTA slurry (Qiagen) was added to 5 ml of cleared lysate and Materials and Methods mixed gently by shaking at 4°C for 1 h. The slurry was packed into Plasmid Constructions. A TAT-encoding fragment was generated a column and washed with 10 bed volumes [100 mM ͞ ͞ ͞ by PCR, using primers NPTatU and NeuUS. This fragment was NaH2PO4 10 mM Tris (pH 8.0) 300 mM NaCl 20 mM imida- ͞ cloned into the pTriEx-1 vector (Novagen) via NcoI and XhoI zole]. Recombinant protein was eluted [100 mM NaH2PO4 10 restriction sites, resulting in pTriEx-TAT-U-H (F.E. and K.R., mM Tris (pH 8.0)͞300 mM NaCl͞250 mM imidazole] and either unpublished data). This vector encodes, in addition to TAT, a dialyzed against appropriate media (see below) for immediate second protein fragment designated as ‘‘U,’’ which was not used use or frozen at Ϫ20°C for storage up to 8 weeks without in this study. The corresponding DNA fragment was deleted by significant loss of activity. Protein concentrations were measured digestion with SpeI and subsequent religation, resulting in using Bradford reagent (Sigma) and͞or Warburg method. Cre pTriEx-TAT-H. The TAT-encoding fragment was removed, activities were determined by a cell-free assay as described (31, resulting in pTriEx-H by PstI digestion. To obtain pTriEx-CH,a 33), except for our use of 250 ng of substrate DNA (33). For Cre-encoding PCR product was generated from the template quantification reactions were transformed into E. coli and pPGK-Cre-bpA (32) by using primers 5H3cre and 3X1cre. This colonies counted. A commercially available Cre protein (New fragment was subsequently cloned into the pTriEx-H by using England Biolabs) was used as a standard. HindIII and XhoI sites. To generate pTriEx-NCH, a NLS-Cre- encoding fragment was amplified using primers 5H3NLScre and SDS͞PAGE and Immunoblot Analysis. Samples were separated by 3X1cre and cloned as above. To construct pTriEx-FNCH, an- SDS͞PAGE (10%), transferred to nitrocellulose membranes nealed oligonucleotides 5PstFGF and 3PstFGF were cloned into (Amersham Pharmacia), and probed with anti-Cre (Novagen) or the PstI restriction site of pTriEx-NCH. To generate pTriEx- anti-Penta-His (Qiagen) antiserum. Anti-Cre was incubated HTNC, a NLS-Cre-STOP encoding PCR product was generated 1:5,000 in PBS containing 5% dry milk, and anti-Penta-His was using primers 5H3NLScre and 3creStopXho and subsequently used 1:2,000 in PBS containing 3% BSA. Blocking was carried cloned into pTriEx-TAT-H via HindIII and XhoI restriction out in corresponding buffers without antibody for1hatroom sites; the His tag was introduced by cloning annealed oligonu- temperature.
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