Phage particle-mediated gene transfer to cultured mammalian cells

Targeting choroid plexus epithelia and ventricular ependyma for drug delivery to the central nervous system

BACKGROUND

Because the choroid plexus (CP) is uniquely suited to control the composition of cerebrospinal fluid (CSF), there may be therapeutic benefits to increasing the levels of biologically active proteins in CSF to modulate central nervous system (CNS) functions. To this end, we sought to identify peptides capable of ligand-mediated targeting to CP epithelial cells reasoning that they could be exploited to deliver drugs, biotherapeutics and genes to the CNS.

METHODS

A peptide library displayed on M13 bacteriophage was screened for ligands capable of internalizing into CP epithelial cells by incubating phage with CP explants for 2 hours at 37C and recovering particles with targeting capacity.

RESULTS

Three peptides, identified after four rounds of screening, were analyzed for specific and dose dependent binding and internalization. Binding was deemed specific because internalization was prevented by co-incubation with cognate synthetic peptides. Furthermore, after i.c.v. injection into rat brains, each peptide was found to target phage to epithelial cells in CP and to ependyma lining the ventricles.

CONCLUSION

These data demonstrate that ligand-mediated targeting can be used as a strategy for drug delivery to the central nervous system and opens the possibility of using the choroid plexus as a portal of entry into the brain.

Specific factors are required for kinase-dependent endocytosis of insulin receptors

Mouse B82 cells that support high affinity saturable endocytosis of epidermal growth factor receptors (EGFR) exhibited only low rates of nonsaturable internalization of insulin receptors (InsR). To investigate the defect in endocytosis of InsR in B82 cells, we examined the role of sequence motifs and tyrosine kinase, the two receptor components shown to be required for efficient saturable endocytosis of InsR in Rat 1 cells. Placement of residues encoded by exon 16 of the InsR onto an EGFR truncated to residue 958 restored EGF-induced internalization of this mutant receptor indicating that the sequence codes in exon 16 are recognized by B82 cells. To determine whether the kinase function could be provided in trans, a B82 cell expressing both receptors was established. EGF-activated EGFR kinase was not able to restore insulin-dependent rapid endocytosis to InsR. However, fusion of untransfected Rat1 cells with InsR-expressing B82 cells enabled rapid endocytosis of InsR, indicating that the internalization defect can be complemented. These results indicate that, although internalization codes can function in the context of other receptors, activation of tyrosine kinase receptors requires an additional specific component.

p lambda Zd39: a new type of cDNA expression vector for low background, high efficiency directional cloning

We have developed a new type of bacteriophage lambda vector which provides a strong biological selection against non-recombinants that is independent of the sequences immediately surrounding the cloning site. This system, which we call 'selective substitution', is ideally suited for cDNA expression vectors where it is necessary to flank the cDNA insert with sequence elements (promoters etc.) required to produce a biologically active mRNA in vivo. Selective substitution is a general method, which may be applied to many types of vectors. In this report, we have specifically applied selective substitution to the construction of a new mammalian retrovirus expression vector. The level of background obtained with this vector (that is, the number of plaques obtained when the vector is ligated in the absence of insert DNA) is 0.02% when compared to ligation with restriction fragments and 0.1% to 0.4% when compared to ligation with newly synthesized cDNA. These features have allowed us to easily and efficiently generate several large cDNA libraries using total and size selected cDNA.

Amplification of a long sequence that includes a processed pseudogene for elongation factor 2 in the mouse

Quantitative Southern blotting analysis has demonstrated that mouse cells contain about 70 copies per haploid genome of a DNA sequence related to the gene for elongation factor 2. The restriction maps of seven cosmids that each carry one copy of the EF2-related sequence (MER) and nucleotide sequences of MERs were highly conserved among the cosmids. Data obtained by such analyses suggest that MERs were produced by the integration of one copy of MER derived from poly(A)+ mRNA for EF2 into a specific site in the mouse genome, with subsequent amplification of MER together with its large flanking sequences during the evolution of the mouse. Furthermore, it appears that the size of each repeating unit is more than 60 kb. Analysis by pulse-field gel electrophoresis suggested that multiple copies of a repeating unit of more than 400 kb (or two units) are clustered at a specific site (or each specific site) in the mouse genome.

Phage particle-mediated gene transfer of recombinant cosmids to cultured mammalian cells

An efficient procedure for the introduction of recombinant cosmids into cultured mammalian cells consists of the following steps. Cosmids were packaged, in vitro, into lambda phage particles and transduced into Escherichia coli hosts lysogenized with thermo-inducible lambda c Its phage. The introduced cosmids were repackaged into phage particles in the thermo-induced hosts. The efficiency of such in vivo cosmid packaging was further improved by construction of pTC vectors that carried three cohesive end sites (cos) of phage lambda, arrayed in tandem. Two types of cosmids, in almost equal numbers (i.e., cosmids with one cos and cosmids with two cos), were obtained from a cosmid library constructed with pTC vectors. The efficiency of packaging in vivo of cosmids with two cos, was found to be 7-20 times higher than that of corresponding cosmids with only one cos. Use of a high-copy-number derivative of pTCl further improved the phage yield by 20- to 30-fold. The packaged cosmids, which carried the thymidine kinase-encoding gene of herpes simplex virus type 1 as a selective marker, were introduced into mouse Ltk- cells with an efficiency of 10(-5), by the phage transfer method [Ishiura et al., Mol. Cell. Biol. 2 (1982) 607-616].

A recB recC sbcB recJ host prevents recA-independent deletions in recombinant cosmid DNA propagated in Escherichia coli

Segments of DNA are deleted from recombinant cosmid DNAs with high frequency during propagation in standard recA Escherichia coli hosts. An attempt has been made to derive an appropriate strain of E. coli, suitable for cosmid cloning, in which such deletions do not occur. We examined the effects of a series of host recombinational mutations on the deletion process, using six independent recombinant cosmids that carry inserts of mouse, Chinese hamster, or human DNA. Various E. coli host cells carrying the recombinant cosmids were cultured serially in liquid medium, and the recombinant cosmid DNAs were extracted from the host cells and analyzed by agarose gel electrophoresis and by gene transfer of the DNAs into cultured mammalian cells. Of the mutations examined, only a recB recC sbcB recJ (or recN) quadruple combination of host mutations prevented the deletion of DNA segments. The recombinant cosmid DNAs propagated in E. coli hosts that carried this combination of mutations were functionally as well as structurally intact. We propose that the recJ (and/or recN) gene is involved in some aspect of the events that lead to deletions of cosmid DNA in a recB recC sbcB genetic background.

Efficient simplified cosmid cloning: construction and characterization of cosmid vectors that carry the two cohesive end sites of lambda phages arrayed in tandem

We constructed a series of cosmid vectors that carry the two cohesive end sites (cos) of lambda phage, arrayed in tandem, which enabled us to clone fragments of genomic DNA of up to 50 kb without a vector background. An equimolar mixture of the left and right vector arms of equal length was prepared from the vector DNA, simply by treating the DNA sequentially with three enzymes, restriction enzyme PvuII, alkaline phosphatase, and restriction enzyme BamHI (or BglII), without purification by agarose gel electrophoresis. After phenol extraction and ethanol precipitation, the equimolar mixture of the vector arms, which carried a single cos oriented from left to right, was directly ligated with insert DNA without further manipulation. We established conditions for cosmid cloning, using two kinds of DNA fragment of 40-50 kb, prepared from mouse L cell genomic DNA, as insert DNAs, namely, three cloned BamHI fragments and Sau3AI fragments, size-selected on a sucrose density gradient. The most important parameters affecting the cloning efficiency were the quality of the insert DNA and the molar ratio of the insert and vector arms. We achieved cloning efficiencies of 3.6 X 10(6)-1.3 X 10(7) colony forming units (cfu)/micrograms of insert DNA and 1.7 X 10(5)-1.0 X 10(6) cfu/micrograms of insert DNA, using the cloned BamHI fragments and the Sau3AI fragments, respectively. We examined more than 5000 clones and found that they all contained insert DNA.

Constitutive expression of c-fos antisense RNA blocks c-fos gene induction by interferon and by phorbol ester and reduces c-myc expression in F9 embryonal carcinoma cells

To address the role of c-fos proto-oncogene we constructed a plasmid that allows constitutive expression of RNA complementary to c-fos mRNA, and stably introduced this plasmid into F9 embryonal carcinoma cells. Some F9 clones expressing c-fos antisense RNA had a reduced basal level of c-fos mRNA, and were unable to induce a c-fos mRNA as well as its protein when stimulated with phorbol ester or with interferon (IFN). Nevertheless, the ability to induce major histocompatibility class I genes following IFN treatment was not impaired in these clones. Clones expressing c-fos antisense RNA grew as rapidly as control F9 cells, and underwent differentiation after retinoic acid treatment. Unexpectedly, constitutive expression of c-myc mRNA was reduced on average by 10-fold in clones expressing c-fos antisense RNA. However, expression of the p53 gene and heat shock gene hsp 70 was not affected in these clones, indicating the existence of a specific regulatory linkage between c-fos and c-myc genes. Cycloheximide treatment led to induction of a large amount of c-fos mRNA in clones expressing c-fos antisense RNA as well as in control F9 clones. The amount of c-fos antisense RNA was also increased by cycloheximide treatment. We postulate that c-fos antisense RNA blocks expression of the endogenous c-fos gene by accelerating the degradation of c-fos mRNA and that cycloheximide treatment interferes with this degradation.

High-efficiency transformation of mammalian cells by plasmid DNA

We describe a simple calcium phosphate transfection protocol and neo marker vectors that achieve highly efficient transformation of mammalian cells. In this protocol, the calcium phosphate-DNA complex is formed gradually in the medium during incubation with cells and precipitates on the cells. The crucial factors for obtaining efficient transformation are the pH (6.95) of the buffer used for the calcium phosphate precipitation, the CO2 level (3%) during the incubation of the DNA with the cells, and the amount (20 to 30 micrograms) and the form (circular) of DNA. In sharp contrast to the results with circular DNA, linear DNA is almost inactive. Under these conditions, 50% of mouse L(A9) cells can be stably transformed with pcDneo, a simian virus 40-based neo (neomycin resistance) marker vector. The NIH3T3, C127, CV1, BHK, CHO, and HeLa cell lines were transformed at efficiencies of 10 to 50% with this vector and the neo marker-incorporated pcD vectors that were used for the construction and transduction of cDNA expression libraries as well as for the expression of cloned cDNA in mammalian cells.

High-efficiency transformation of mammalian cells by plasmid DNA

We describe a simple calcium phosphate transfection protocol and neo marker vectors that achieve highly efficient transformation of mammalian cells. In this protocol, the calcium phosphate-DNA complex is formed gradually in the medium during incubation with cells and precipitates on the cells. The crucial factors for obtaining efficient transformation are the pH (6.95) of the buffer used for the calcium phosphate precipitation, the CO2 level (3%) during the incubation of the DNA with the cells, and the amount (20 to 30 micrograms) and the form (circular) of DNA. In sharp contrast to the results with circular DNA, linear DNA is almost inactive. Under these conditions, 50% of mouse L(A9) cells can be stably transformed with pcDneo, a simian virus 40-based neo (neomycin resistance) marker vector. The NIH3T3, C127, CV1, BHK, CHO, and HeLa cell lines were transformed at efficiencies of 10 to 50% with this vector and the neo marker-incorporated pcD vectors that were used for the construction and transduction of cDNA expression libraries as well as for the expression of cloned cDNA in mammalian cells.

Extrachromosomal recombination in mammalian cells as studied with single- and double-stranded DNA substrates

We have previously proposed a model to account for the high levels of homologous recombination that can occur during the introduction of DNA into mammalian cells (F.-L. Lin, K. Sperle, and N. Sternberg, Mol. Cell. Biol. 4:1020-1034, 1984). An essential feature of that model is that linear molecules with ends appropriately located between homologous DNA segments are efficient substrates for an exonuclease that acts in a 5'----3' direction. That process generates complementary single strands that pair in homologous regions to produce an intermediate that is processed efficiently to a recombinant molecule. An alternative model, in which strand degradation occurs in the 3'----5' direction, is also possible. In this report, we describe experiments that tested several of the essential features of the model. We first confirmed and extended our previous results with double-stranded DNA substrates containing truncated herpesvirus thymidine kinase (tk) genes (tk delta 5' and tk delta 3'). The results illustrate the importance of the location of double-strand breaks in the successful reconstruction of the tk gene by recombination. We next transformed cells with pairs of single-stranded DNAs containing truncated tk genes which should anneal in cells to generate the recombination intermediates predicted by the two alternative models. One of the intermediates would be the favored substrate in our original 5'----3' degradative model and the other would be the favored substrate in the alternative 3'----5' degradative model. Our results indicate that the intermediate favored by the 3'----5' model is 10 to 20 times more efficient in generating recombinant tk genes than is the other intermediate.

Electroporation for the efficient transfection of mammalian cells with DNA

A simple and reproducible procedure for the introduction of DNA into mammalian cells by electroporation is described. The parameters involving the cells, the DNA, and the electric field are investigated. The procedure has been applied to a broad range of animal cells. It is capable of transforming more than 1% of the viable cells to the stable expression of a selectable marker.

Extrachromosomal recombination in mammalian cells as studied with single- and double-stranded DNA substrates

We have previously proposed a model to account for the high levels of homologous recombination that can occur during the introduction of DNA into mammalian cells (F.-L. Lin, K. Sperle, and N. Sternberg, Mol. Cell. Biol. 4:1020-1034, 1984). An essential feature of that model is that linear molecules with ends appropriately located between homologous DNA segments are efficient substrates for an exonuclease that acts in a 5'----3' direction. That process generates complementary single strands that pair in homologous regions to produce an intermediate that is processed efficiently to a recombinant molecule. An alternative model, in which strand degradation occurs in the 3'----5' direction, is also possible. In this report, we describe experiments that tested several of the essential features of the model. We first confirmed and extended our previous results with double-stranded DNA substrates containing truncated herpesvirus thymidine kinase (tk) genes (tk delta 5' and tk delta 3'). The results illustrate the importance of the location of double-strand breaks in the successful reconstruction of the tk gene by recombination. We next transformed cells with pairs of single-stranded DNAs containing truncated tk genes which should anneal in cells to generate the recombination intermediates predicted by the two alternative models. One of the intermediates would be the favored substrate in our original 5'----3' degradative model and the other would be the favored substrate in the alternative 3'----5' degradative model. Our results indicate that the intermediate favored by the 3'----5' model is 10 to 20 times more efficient in generating recombinant tk genes than is the other intermediate.

Negative regulation of the major histocompatibility class I gene in undifferentiated embryonal carcinoma cells

Murine embryonal carcinoma F9 cells, which do not express appreciable levels of major histocompatibility complex (MHC) class I mRNA, start to express the mRNA and proteins upon differentiation induced by retinoic acid (RA). To investigate the molecular mechanism of this regulation, we examined in F9 cells transient expression of the chloramphenicol acetyltransferase (CAT) gene directed by the 5' flanking region of a MHC class I gene, H-2Ld. The native 1.4-kilobase H-2Ld 5' upstream region gave very low CAT activity in undifferentiated F9 cells. Deletion between positions -210 and -135 relative to the cap site resulted in a 4- to 5-fold increase in CAT activity as compared with constructs containing the region. However, all of these constructs, regardless of the deletion, expressed comparable CAT activity in differentiated F9 cells. These data suggest the presence of a negative cis-acting element that is under developmental control. Further analysis revealed that the sequence conferring the negative regulation resides between positions -195 and -161. This region, highly conserved among the MHC class I genes, is found to be capable of increasing CAT activity in NIH 3T3 cells that express the class I genes constitutively. Further, this regulatory sequence, when connected to the simian virus 40 promoter, produced repressive and enhancing effects in F9 and NIH 3T3 cells, respectively. Based on these results, we suggest that the expression of MHC class I genes during development involves switching from negative to positive regulation dictated by the class I regulatory element located between positions -195 and -161.

Bacteriophage lambda vector for transducing a cDNA clone library into mammalian cells

We have developed a bacteriophage lambda vector (lambda NMT) that permits efficient transduction of mammalian cells with a cDNA clone library constructed with the pcD expression vector (H. Okayama and P. Berg, Mol. Cell. Biol. 3:280-289, 1983). The phage vector contains a bacterial gene (neo) fused to the simian virus 40 early-region promoter and RNA processing signals, providing a dominant-acting selectable marker for mammalian transformation. The phage DNA can accommodate pcD-cDNA recombinants with cDNA of up to about 9 kilobases without impairing the ability of the phage DNA to be packaged in vitro and propagated in vivo. Transfecting cells with the lambda NMT-pcD-cDNA recombinant phage yielded G418-resistant clones at high frequency (approximately 10(-2]. Cells that also acquired a particular cDNA segment could be detected among the G418-resistant transformants by a second selection or by a variety of screening protocols. Reconstitution experiments indicated that the vector could transduce 1 in 10(6) cells for a particular phenotype if the corresponding cDNA was present as 1 functional cDNA clone per 10(5) clones in the cDNA library. This expectation was confirmed by obtaining two hypoxanthine-guanine phosphoribosyltransferase (HPRT)-positive transductants after transfecting 10(7) HPRT-deficient mouse L cells with a simian virus 40-transformed human fibroblast cDNA library incorporated into the lambda NMT phage vector. These transductants contained the human HPRT cDNA sequences and expressed active human HPRT.

Bacteriophage lambda vector for transducing a cDNA clone library into mammalian cells

We have developed a bacteriophage lambda vector (lambda NMT) that permits efficient transduction of mammalian cells with a cDNA clone library constructed with the pcD expression vector (H. Okayama and P. Berg, Mol. Cell. Biol. 3:280-289, 1983). The phage vector contains a bacterial gene (neo) fused to the simian virus 40 early-region promoter and RNA processing signals, providing a dominant-acting selectable marker for mammalian transformation. The phage DNA can accommodate pcD-cDNA recombinants with cDNA of up to about 9 kilobases without impairing the ability of the phage DNA to be packaged in vitro and propagated in vivo. Transfecting cells with the lambda NMT-pcD-cDNA recombinant phage yielded G418-resistant clones at high frequency (approximately 10(-2]. Cells that also acquired a particular cDNA segment could be detected among the G418-resistant transformants by a second selection or by a variety of screening protocols. Reconstitution experiments indicated that the vector could transduce 1 in 10(6) cells for a particular phenotype if the corresponding cDNA was present as 1 functional cDNA clone per 10(5) clones in the cDNA library. This expectation was confirmed by obtaining two hypoxanthine-guanine phosphoribosyltransferase (HPRT)-positive transductants after transfecting 10(7) HPRT-deficient mouse L cells with a simian virus 40-transformed human fibroblast cDNA library incorporated into the lambda NMT phage vector. These transductants contained the human HPRT cDNA sequences and expressed active human HPRT.

Recombination in mouse L cells between DNA introduced into cells and homologous chromosomal sequences

In this paper, we show that DNA added to mouse L cells by the calcium phosphate method can be inserted into the genome of those cells by homologous recombination. The insertion event is detected because it reconstructs a functional thymidine kinase (tk) gene from two defective genes that share 320 base pairs of homology. One of the genes is missing its 5' portion (tk delta 5') and is in the cell's chromosome, and the other is missing its 3' portion (tk delta 3') and is in the introduced DNA. Gene reconstruction by homologous insertion is relatively inefficient; approximately one Tk+ transformant is produced per 10(6) cells per 4 micrograms of added tk DNA, a frequency of about 10(-5) that of normal tk gene transformation. The Tk+ transformants produced by homologous recombination contain Sma I and Pvu II fragments that are diagnostic of the intact tk gene, contain a herpesvirus-specific thymidine kinase activity, and can transfer the Tk+ phenotype to Tk- cells by DNA-mediated gene transfer. Two surprising observations made in the course of these studies were that only 1 of 10 Tk- cell lines containing defective tk genes could be transformed to Tk+ by homologous insertion of the complementary defective tk gene and that relatively little illegitimate insertion of introduced tk DNA into cellular DNA was detected in those cells that were transformed to Tk+ by homologous recombination.

A vehicle for DNA transfer and for recovery of transferred genes: lambda Charon phage-pBR322 hybrid

Recombinant Charon 4A phages accommodating the Herpes simplex virus (HSV-1) thymidine kinase (tk) gene, the ampicillin-resistance (ApR) gene, and the replication origin of pBR322 were constructed. The phage DNA was introduced into mouse Ltk- cells by a free DNA transfer method or phage-mediated DNA transfer method [Ishiura et al., Mol. Cell. Biol. 2 (1982) 607]. Analyses of the physical state of the transferred DNA in the recipient cell genome showed that a DNA fragment as long as 12.7 kb was integrated intact into 67% and less than 40% of the Ltk- transformant cells by phage-mediated DNA transfer and by free DNA transfer, respectively. We also developed a new rapid method for recovery of the transferred gene from the Ltk+ cell into Escherichia coli; the method depends on the fact that the recombinant lambda phage carrying the ApR gene and replication origin of pBR322 transduces lambda-lysogenic bacteria to ApR and is maintained as a plasmid. Using this method the HSV-1 tk gene from one Ltk+ transformant was rapidly and successfully recovered without any rearrangement of the target sequence.